I'm currently training as a driver in London, and would advise as follows.
The typical trainee driver age range in my TOC is late twenties upwards, with the majority in their 30s or older, from a range of backgrounds and walks of life. What they have in common is that they have passed the tests to get in, and have a decent work history behind them. I'm aware of one chap who is 22 but he is the exception rather than the rule.
Academics aren't important, but remember as part of the process you will have to undergo rigorous aptitude testing which, with the exception of the group boredon "dots" test, cannot be practiced for. In my case I have a degree and this was not of great importance, although the interviewers commented favourably on it. I still had to pass the same tests as everyone else.
Part of this testing is the multi-modal interview which is competency based and will require you to draw from your past experiences to demonstrate competencies required for the driver role. I would say get as much work / hobby experience as possible in whatever field - customer service, team working and any kind of "safety critical" responsibilities are particularly valued.
If you apply before gaining much experience you may pass the aptitude tests but fail the interview, and you only get two attempts at the process before you are barred for life. Worth remembering and ensuring you develop good experience before applying. You have a few years ahead of you to do this, and also think of a back up plan in case you change your mind about train driving or don't manage to pass the assessments.
Best of luck with it.