The only official age limit is on the lower end, with most saying you need to be 20.5 years old at time of application as drivers need to be 21+. Above that age the only caveat is being able to pass a medical and the entry tests which probably won’t be possible if you’re 75 years old.
Getting onto the apprenticeship is not easier than any other route into driving, it’s the exact same assessments and pass mark as non-apprentice routes. It’s not really a traditional apprenticeship in the sense that they’d take on a “lower calibre” (can’t think of a better term, don’t mean offence by that!) employee and train them up to the standard - they still only take on people who can pass all the assessments to a high level and then train them just the same as any other train driver.
Be aware that if you’ve failed once you only have one more chance, so only sit the tests if you’re absolutely confident you can pass or you’re confident you’re as ready as possible.