Obviously no one can tell you whether or not that is possible - it really does depend on what sort of person you are, how well you study alone and how well you can self-motivate - but I've just finished taking accelerated chemistry and I would have found it a lot harder without the support of my teachers. And I might have coped with taking a second subject, but definitely not four. It is a huge amount of work to cram into one year and you'd have a LOT of exams to take in January and June.
You also have to think about how you'd divide your time between the two years. I did AS and A2 at the same time, which effectively gave me a full year for the A2 part of the course (which I really needed, as the second year is a lot more difficult). This obviously isn't how the course was meant to be learnt and I struggled in the first few months because I hadn't covered the AS material which is built upon in A2. My teachers were able to fill in the gaps a bit before I caught up on the AS work, but if you're learning from home you'd probably have to do the AS material first, and this would leave you with only half a year to cover a year's worth of material in four subjects, which is really not a lot of time, especially if you're doing sciences.
Personally, I'd take two years. You're much more likely to succeed. If you try to take all four subjects in a year and fail or do really badly, you'd have just wasted a year and it won't look good on your record. And please don't worry about being too old - I probably won't graduate until I'm 28-30 because of the path I've taken so I know how you feel, but there are a lot of mature students at university (including many doing medicine) and it really shouldn't make any difference to universities or other students.