I am almost 21 but I have been offered drinks in FC and on international flights since i've been about 17 or so. THe FAs don't usually seem to care although if you are flying alone, they may be more likely to question you. On non US based carriers (AF, BA, etc), they don't even think twice about serving you.

The only thing I am questioned about on airplanes is if I am over 15 when I sit in the emergency exit rows- even if i'm scruffy after a busy week.