I don't know there is a maximum age. I read the start of a journey fic where the main character was 21, and it seems like it would have worked. By the same token, you could leave on a Pokemon journey when you're fifty and it would be fine. All you really need to have is a good reason why they haven't gone before - which isn't really as hard as some would have you believe. Some people decide to commit themselves fully to school so they can study to become a lawyer, others are pressured into taking up the family business, others just plain decide they don't want to be Trainers before changing their mind several years down the track.