Just for your interest, in my case i was accepted on a sponsorship earlier this year at the age of 20 - so obviously the airline trusted me enough with their equipment. But there are some points to note:
1)By the time i wouldve completed training and been on the line i wouldve been 21/22.
2)I am a uni student and was accepted at the end of my second year (i wouldve had to forfiet my final year), which certainly helped my application.
3) The above points are past tense because sept 11th came along and has held things back a little!
But the moral of the story is, if you can demonstrate that you're mature enough during the selection process, then your age shouldnt hold you back.
Good luck.