What someone needs to do is compare all the models of each gender by height to get an idea of in-game ages.
Lucas
Height: 143 cm (4'7")
Weight: 38 kg (83.8 lbs)
Dawn
Height: 140 cm (4'6")
Weight: 34 kg (75.0 lbs)
Ethan
Height: 150 cm (4'11")
Weight: 40 kg (88.2 lbs)
Lyra
Height: 152 cm (4'11")
Weight: 41 kg (90.4 lbs)
From this, we can see that Ethan and Lyra are probably a year older than Dawn and Lucas. Red, Leaf, Brendan, and May also have height comparisons but no numbers given, but to compare, it means that there must be some internal number that's being referenced.
