I've done a couple of legs in Business this year, loads in Domestic First, and I've never worn anything other than shorts/jeans and a t/polo shirt.
I'm 20, and whilst I get funny looks sometimes (usually envious ones from those eye......g me as they walk to the back
) noone has ever said anything, and the FAs usually seem to pay me a bit of extra attention over everyone else in a shirt/suit. Seemingly being the only non American most of the time also does help though.
This is especially so when they work out you are travelling on your own and you arent just there because you joined Daddy on his business trip to London - or whatever the little girl on the AA ad is doing!