I'm going to be honest. You're worried about your crew being viewed as young inexperienced kids and I find that worry legitimate. Mussonman is definitely right about how young people are viewed in America.
Although I am not young myself, I have to admit I look very young for my age. While some friends say it is a blessing, I find that in the business (film) world it is a curse. In the acting world, there seems to be a certain jealousy towards young new talent. In the directing world, I've found there to be a favoritism towards older artists. "He looks older and he's holding the camera, using his hands so much to explain something, he must be brilliant and experienced!"
For me, I've felt that people try to pull the wool over eyes. Take positions they feel like they're more suited to have. You ask them why and they have no reply. The fact that you're young and in the director's chair doesn't look right to them. It's not the painted picture of a director they have in their head.
Directing on some sets, during downtime, sometimes it slips out what my age is. They're all surprised at how old I am since I look so young. It's amazing and just sickening at how their attitude changes once they realize you're older. It's nice that they're giving me respect "now," but why? I'm directing and leading the same way as the beginning of the shoot, but because they know I'm older, suddenly they respect my ideas more? You should respect people based on their actions, not their look. Keyword there is "should," but many people still judge based on looks.
So my advice is to act as professionally as possible. When people doubt you, you have to over prepare. You gain more by erring on the side of professionalism than being too informal. Yes you can keep things friendly and light, just don't forget that you're trying to make it and you want this short/feature/episode to look as great as possible.
Hey you look old, then you can get away with goofing off a bit. But if you look like a teen and do something that gives them even a hint that you're goofing off...... you could trigger this subconsious thought in their head that "Oh I'm on one of these projects."
That doesn't mean to be a dictator or overly commanding. But just remember you have to impress them with what you know. Most people will tell you otherwise, but sad fact is that if you look young you're going to have show them how serious you are about making great films. Remember, you don't look like most directors they've worked with, so you have to do something to shake those preconceived notions out of their head and get them thinking "Oh wow, this is a serious production here." Basically do the research, take responsibility, be as professional as possible, and let your experience show.
Other cheat sheets are equipment, budget, how you write the audition notices, way you dress, and body langage and ettiquette. I'll try to explain these. Any of these you can do will help you out with appearing more professional. If the equipment looks professional, instead of "wonky" it builds a sense of professionalism. There are many DIY tutorials on the internet for camera rigs and other accessories. After the completion of each rig they mention optional things you can do, like spray painting it to one solid color, to bolt down every part so theres no risk of it coming apart, etc. For good reason. You don't want pieces falling off, or a boom pole that looks like a broken broom handle. Present yourself and your equipment professionally. You dont brag, but they'll be seeing the equipment a lot anyway. If you were 40 years old, they wouldn't second guess your rig coming apart and might just laugh it off as a would-be out take or blooper. You're young and your boom pole falls apart and they're painting a picture about the production already. I remember when one of our actors got to our set. He was so surprised at how professional it looked. I think he thought it was going to be a wonky unprofessional low budget film. It was low budget, but it showed him it wasn't going to be a wonky production. It wasn't until a few days later that during lunch he found out my age and was dead surprised. Just put "equipment" in the category of keeping things professional and letting them see it naturally on set.
For budget, there's a lot that falls into this category. If they get the sense that the production has a decent budget they tend to take it seriously. You don't need to have a lot of money, but having an impressive set, equipment, props, wardrobe, and such can remind them of how seriously you're taking this and the amount of preparation you put. I'm not sure if you're paying them anything, but they've probably been on a few sets that weren't paying anything that just ended up being shoddy productions. Keep your equipment, your sets, clean and organized. It can make a $0 budget production look like amazing demoreel material for your actors.
The way you dress can help them to see you as "older" and professional. I've seen older directors come to sets dressed in what looks like pajamas! Barefoot and all. Maybe they get away with it because they look older (maybe they don't because it's such an extreme example.) When you wear something that looks professional, it starts to say the right things about you to the actors and crew. You're young and they're expecting you to wear something a typical teen would wear on a day out. Suprise them and wear something that rings, "professional." When you wear something like that, it also carries through in your body language and you carry yourself a bit different as well. I mentioned body langage and ettiquette above. You don't have to be too meticulous about this or think too much into it. I had an acquaintance on set who was very serious about film. However, his body language said otherwise. I think he was overly lax and would just kick up and put his feet on everything. He would use so much slang and the actors didn't know what he meant half the time. Remember how you speak and how unprofessional cursing and some slang can sound when you look so young. (Well younger than what they've been working with.) Just remember not to slouch and not invade people's personal space by throwing your feet practically on their lap. I think sometimes teens have a different sense of humor than the older crowd. I worked with a few young people on set, and the types of jokes they made just seemed offensive and insulting to the actors even though they were probably just trying to entertain and make them laugh. Don't let them see you as filmmaking teens that don't take this seriously. Take care of them and ask if they need anything. It's just something that happens on professional sets and makes them want to work with you again (Not only do you have your directing game down, you have it much so, that you have extra clarity to ask them and take care of their needs.)
The last is the most important in my opinion. The casting call that you put out is a great way for them to get the right idea about you from the get-go. It's amazing how I've seen audition notices from professional casting agents starting with "Casting for a feature-film directed by an Emmy nominated name director." And the rest of the audition notice looked like it was typed by someone who got back from work, had a few drinks, forgot to type that last audition notice, ran home, and typed the first things that came to mind. It really did start with "Casting for a feature film directed by an Emmy nominated name" but the rest.... let's just say spelling and grammar were the least of the worries. It's obvious it wasn't the actual director placing the notice, but I think most directors would want that changed. Most of us aren't Oscar or Emmy nominated, so we cannot afford to try to get away with poor casting notices. Look through a few audition notices and look for the most professional of the bunch. Study it a bit before making your own. The audition notice is a great way for the actors to check YOU out as the director. Besides selling your short or film as something worthwhile, you also might want to mention your company or a link of previous work. If you mention your company name, most actors that are interested will want to google it and see what kind of director they're working with. It doesn't have to be a perfect audition notice, but you want to make it sound more professional instead of too informal. Be honest, but let it be a chance for you to make an good early impression with them. When they DON'T see you as part of the same category as the other naive, unprepared, young filmmakers (that they've surely auditioned and worked with before) it leads to good things happening on set. And don't freak out with so much of these points, since they may have worked with a very young crew before and walked away with great experiences or getting possibly their favorite material of their demo reel!
Although this seems like many steps, or like a very specific tutorial, it's more me trying to reverse-engineer certain things I've done throughout the years to let them realize I'm not as young and informed as they think. Much of this will probably come naturally to you through out the years. Anyone that starts out young and wants to work with older talent will tend to go through this.