There isn't a simple answer to this. In my crew room we have almost as many different backgrounds as we have pilots - ex military guys (both fast jet and truckies), ex flying instructors, teachers, policemen, engineers - and in my case nearly 15 years as a shipping monkey.
The critical thing in this game is to have a workable plan 'B' (and ideally plans 'C' and 'D' as well). That's where education, and a degree in particular, can serve you well - it can open doors that would otherwise remain closed. The degree subject doesn't matter from an aviation point of view (airlines really aren't interested) but it goes without saying that a marketable subject (e.g. engineering or business studies) will be more useful in the outside world than sports science or philosophy. That said, a degree is a big commitment in time, effort and money, so I would only recommend it if you are academically minded and interested in the subject - not just as a back-up in case you lose your medical.
Another excellent plan 'B' would be qualifying as an electrician or plumber - trades that will always be in demand and offer the possibility of flexible or part-time working while you train as a pilot. Even something like a HGV licence could come in useful.
In terms of A-levels, physics is the one that stands out as most useful when you come to study the ATPLs. I failed A-level maths, but a working knowledge of the three times table is perfectly adequate for flying modern airliners.
Regarding age, again there's no such thing as a 'best time' to start, everyone's circumstances are different. As a teenager, you have all the time in the world - many people change careers and get into flying in their thirties (I got my first job aged 36). Especially in the current climate, I would strongly advise against throwing a pile of cash at flying training (wherever you get it from and whatever the flying schools tell you). Take your time, get a job, and pay for your training as you go along.
There's a whole generation that's just starting to discover the perils of being in debt up to their eyeballs . . .
