No age limit. I've flown with guys who said they didn't find out their FO was under 21 until the overnight, when they went out to the bar to grab a drink. And just to be clear, I'm not bashing anybody for being young in this industry. If you can get your ratings, and your time in by the time your 20, more power to you. The military seems to trust 18 year old kids to fly for them, so I don't see any problem with it.