my guess is 16-18. shauna is probably 10-13, trevor 10-12 & tierno 11-13
regardless the main character has to be 16 or older & is a few years older then shauna given how she tends to look up to him/her. also grace seemed to have no qualms about outright wondering if sycamore's letter was a love letter. most young children would be unsettled by that. though that variable is something of a wild card as we know not if the players father is dead is dead or divorced her. odds are he's dead, i don't think getting sent flying by a ryhorn can be very good for one's health.
Considering B/W's player characters' ages were a significant change from all previous generations at that point (age 10 all the way to Generation IV), it always looked to me X/Y's player characters were at least 12 or 13, though 14 certainly sounds like it could be in the ballpark.
red is 11 in r/b/g/y/fr/lg, odds are blue and leaf are the same age
in g/hg/s/ss/c ethan's supposed to be the same age as silver kris & lyra. there wouldn't be much to this if it wasn't for silver being confirmed to have been on his journey for at least 3 years.
brendan & may are both 12
lucas & dawn don't have a confirmed age
hilbert & hilda where both made to look 16 & are around 14
yet again, nate and rosa don't have a confirmed age, though they seem to be in the made to be 14 but look older grouping.
none of the games or even the spin offs have ever confirmed the player characters age to be 10, its always been an anime only thing to the point of contradicting plot holes...