Scott: Things will have changed a bit since I retired just over 2 years ago but there is still the age limit, which I believe is 21. I was 22 when I started on the railway I had to wait 11 years for my driving job, starting off as an engine cleaner then upgrade to Fireman and eventually to driver. Infact, if I hadn't moved depot once again it would have been 17 years before I would have got my driving job automatically via seniority.
Much depends on what type of train driver you wish to be. We didn't have that problem as we covered all types of train under B.R. but today you will either join a passenger or a freight company. Again, age comes into it therefore you are going to have to wait a while before you even get to the first stage.
In the meantime you could start work outside the railways until you come of age or you could possibly start on the freight side with the inital grade that allows you to move up to a trainman. Trainman, is a good place to strat as you not only learn about the running and shunting side of train work but wiil also work on trains and get to know drivers and how to work together and at the same time learn about what is required for driving.
No matter what your qualifications are you will have to pass the 'computer' test. I can't remember what you call it, but you will have to be tested for speed of thought and angility etc. If you can't get through that test qualifications do not mean much. However, being smarty pants on a computer doesn't mean you have the right stuff to make a driver as there is much more to it than that. Thank goodness computers as we know them weren't around in our days as we would probably never have passed and they would lost a lot of really great drivers, guards etc. The only thing resembling a computer that we knew about was an Abacus, which we were taught maths on at school. That's arithmatic to you. However, when I was 12-13 I was a dab hand at playing with Yo-Yo's winning at marbles and completing jig-saw puzzles so that probably was as close to shoot-em-ups on the computer as you could get back then. Unless you can count playing at Cowboys & Indians in the same way! Been a keen reader was also a great help.
I hope that this gives you some idea what to expect and how difficult it is to reach the goal of becoming a driver. But don't give up. It just takes a bit of patience and determination. I worked with a Bakery company, in the cinema, theatre and a sawmill factory long before I joined the railways, proving that there is another life outside the railways to learn and gain experience from. My generation left school at 15 and without any qualifications therefore you had to push yourself and find a suitable job and gain experience to rise above the everyday type of employment and earn reasonably wages. You don't get it all from a text book or a computer. My first wage was £1.0.6d (£1.2p). If you didn't push yourself you would be stuck on that type of low wage forever. My final wage was around £34k, so didn't do so bad in the long run.
Sorry I can't help you with passenger companies as I know little about how that department start train crew these days. But a trainman would certainly be a worthwhile job to start with.