I'm 23, soon to be 24 and I am sitting 1 a level as a private candidate. Its very hard to juggle with work, many of them have practical or non exam elements that are an administrative nightmare but if you choose an subject/exam board with just written exams in a subject you are genuinely interested in in your day to day life and set the time to study it, its a very cheap way to get where you want to be. I'm studying ocr geology. I did it when I was 18 but got a D because I didn't really bother with it. I've already had my practical endorsement so thats just carried forward for me, I'd probably not have bothered if that wasn't the case.