Once they're past the 2-4yo, baby brain, might-have-a-greenie-moment stage, age doesn't make all that much difference. Breed can (I emphasize CAN lol) make a big difference too. In general I'd take a 5yo been-there-done-that ranch-used QH as a mount for an inexperienced rider over a 14yo OTTB or very forward performance horse any day.
“A bad day of roping beats a good day raking.” ~Buck Brannaman
RIP Ben 04/01/1998 - 02/28/2017