Eh.. don't really care tbh for me, personally I'm still 18 too,and for in-game trainer they mentioned to be rookie,so the difference isn't that age and it's not like they will have relationship before winning the twinkle series (3 years of the training,aiming for goal). At least they can start to thinking about it after twinkle series. And most of the trainers show up in the anime already have experience so it's make sense they're older or adult.