We know wizards live longer than Muggles (hence Dumbledore being well over 100 when he died), so certain people seeming younger than they really are should not be uncommon; it's the stage of their overall life they are in.
What about Harry's kids? We know Teddy is 19 at the time of the epilouge; I would guess Victoire is 17, as James Sirius says Teddy has "come to see her off". Al, of course, is 11, as is Rose; and Lily Luna (and I think Hugo, too) is 9. I think James Sirius is about 13, that just seems like the right age (plus, it makes it two years in between each new Potter kid!) But what about Molly and Arthur's other grandchildren, Victoire's brothers (One of them is Louis and I forgot the other!), Molly and Lucy, Roxanne and Fred (that would be Bill, Percy, and George's kids, in order!)
"You said to us once before," said Hermione quietly, "that there was time to turn back if we wanted to. We've had time, haven't we?"
Pottermore: SeekerSpell89, DawnDream24997
Wand: Cedar/Cypress, Pheonix, 10 3/4 in, Slightly Springy