Have a look at the websites for the various institutes - CIMA, ACA, ACCA and CIPFA.
Bear 1 thing in mind, I started training when I was 34, and never gained back the salary I lost. I may have been unlucky, or trained with a shit firm - I was made redundant during my training contract, which was a breach of contract, but the members of the ICAEW had taken counsel's opinion, so loads of firms did it with impunity in the '90's. Also, my personal pension - the firm didn't provide one, is worth less than 2k per year.
There is normally a graduate conversion course, now called certificate, if you don't have a relevant degree. Your maths degree would give you an exemption from the maths, and possibly the IT paper, if those subjects still exist.
It's worth contacting your local regional office, and speaking to their training advisor, especially which firms will recruit 'older' trainees. Some will, some won't, depends if they think 21 year olds are more malleable. The big 4 possibly won't.
You could always try and get a couple of weeks work experience with a local firm during the school summer holidays. I got an intro to a local accountant via the bank manager - a couple of centuries ago.
If you do ACA, you will have study pack exams every 2 weeks. My firm, we did them Saturday mornings, the big 6, as they were then, did them in the office on a Friday afternoon. They also got a week study leave before going on courses. I used to take holiday.
You could also try someone like Career Analysts to do aptitude tests to see if it might be for you. Bastards told me to become an accountant - should have stayed in IT, with a pension of 35k a year. Hindsight is a wonderful thing.