I effectively left school at 14 with no qualifications at all and went back to college some 4 or 5 years later to get my 'school leaver' qualifications (GCSE's).
So, I was always slightly older than the majority of people in my class. Did I find it a problem? Not really. The only sticking point was when I was at my PhD interview - it was suggested I was a little 'old' to be doing a PhD by one of the interviewers (I was 28-29 years old). Didn't stop me from being accepted thankfully.
Its weird but I kinda just 'fell into science'. After my GCSE's I signed up for a pre-university IT course (BTEC - I don't even think they exist anymore). Hated it, wanted to change to Health Sciences after a week. Only course they had with openings was plain old Science. So, plain old science is what I did.
My degree was in Biomedical Sciences (just so you know Huge, I only got a 2.i as well so I don't consider myself exceptionally bright). I loved the course but loathed the university. After Oxford and Cambridge its considered the best out there in the UK so it was filled with 'rich kids' that failed to get into Oxford and Cambridge (I come from very much a working class background). And, even worse, my course was filled with 'rich kids' who failed to get into medical school (being a medical doctor was never an ambition of mine). In other words - do your homework college/university wise. I knew I was wise to keep away from Oxford and Cambridge (later on I was offered a PhD at Cambridge but turned it down flat) but I should have kept away from the one I did choose as well. Academic excellence shouldn't be your only criteria for choosing a university - in my experience anyway. You only get to do it once - enjoy it. Its not just about getting good grades.
Having learnt my lesson (and by then deciding I wanted to do a PhD in Bioinformatics) I got some funding to do a Masters in IT (yes, I know
) at my local university (a former polytechnic). Way down the pecking order for most subjects but had a world class Maths and Computing department. Its a shame the course was only for a year because it was so much more enjoyable than my undergraduate degree. Also, the university I went to was known for attracting mature students which for me was a bonus.
Don't know if that helps, but there it is anyway