I started flying at 13 (in the US), soloed on my 16th birthday, got my PPL on my 17th, commercial license on my 18th, etc.... It was a passion of mine long before then and continues to be one today.
Although I used to work as a flight instructor, I took my elders' advice and became a lawyer, which I still am today (although the thought crosses my mind at least once a day as to whether I would have been happier going into the RAF and/or commercial flying) but I still avidly fly privately.
If aviation is a passion for you, I would definitely encourage you to pursue it from now. We all know it is expensive but there are many ways of feeding that passion in the early years such as through the RAF air cadets, working/volunteering at an airfield (doing anything you can) and reading every book you can get your hands on to do with the type of aviation you're interested in. MS Flight simulator has also come a long way since I was 13......
Having a passion at your age will give you something to be interested in and strive for. That alone should make you a more interesting and interested individual all round and stand you in good stead whatever you decide to do in later life (it may also help with school and university applications).
More importantly though, an extra-curricular activity such as flying absolutely has to come after your school performance (remember, the CAA can take your pilot's license away from you but nobody can take away your A's at GCSE and A level or a good university degree). Make sure you get the best results as possible at school as this will seriously increase your options later in life.
Good luck!!