I've been thinking. Should I RP an old NCR veteran, or a young man barely able to grow his first chin hairs?

young man

don't see an old grizzled war veteran being like 'hmm nice retirement. I know I'll be a courier as well in this brutal wasteland'

plus as a vet people in the NCR would treat you with more respect (before you're well liked) so for RP reasons in that way no (unless you feel like you can 'forget' the first few missions until you're 'idolised' or w.e

But that's my opinion. In the end you should do whatever you feel you can make a story for, or feel happier about playing. Do whatever you want