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.
That depends on what you are asking I suppose; to travel at all or start travelling. For the former I'd say there is no age limit; some veterans in the games still do battles after all so if there's a motivation for them to continue travelling and training their Pokemon. (As for a stronger canon case, Wes the protagonist in Colosseum is an established trainer already; older teens/young adult and with two Eeveelutions).
And I suppose there's the same deal with trainers starting out, although the reasoning for someone to start older than usual is still a factor in that. Just has to be backed up well really. (And BW is a canon case with the protagonists reportedly older than previous generation games. And the whole Belle/father relationship showing even some parents in the Pokemon world can be overprotective of kids going off to have adventures.)