I thought I'd post this as I was / am in quite a similar situation to yourself and might be able to help you with my own personal experience.
Probably, likewise as yourself - the life-long, childhood dream of becoming an airline pilot etc. has existed.
Unfortunately - I did not do so great at GCSE - but I did go to College to try and get 4 A-Levels - eventually only managed 2 - one of which was Physics and then went to a second-rate Uni (as it was, better now!) to study Physics.
Presently, I am about to start my third year at said Uni, still studying Physics. However at the same time - I have also made the choice to start NPPL (Microlight) training. - Okay, not a PPL / Airline Pilot training / rating but I have decided to take that step nonetheless.
- I looked at doing the 'integrated courses' at Uni i.e a course with training but decided to settle on Physics 'in general' as it has a few more 'fallback' options should something in aviation related industry not materialise.
In my case - while going to Uni adds debt - it has also given me the opportunity to be able to afford flight training. (Okay - it is a 'loan' of money but at a much better rate, and gives me the time (not a 9-5 day job) to be able to attempt it).
I think - regardless of your age - though you are still young after-all!, you should do what you personally want to do - as they say, necessity is the mother of all invention - and as others on this forum have posted - if you want to do something, you will find a way to do it, regardless of the circumstances, cost etc.
If you are committed to be an airline pilot - I would first advise, if you have not already done-so, a medical to ensure you can - otherwise - that is a LOT of time / money / hope to be wasted.
-Partly why I decided to go for a NPPL M - easier to get, but just as fun as real flying though it is cheaper, too!
Age is not really a cut-off for jobs - it is ability / capability / expected performance - okay - you might not retire as a senior captain on a heavy - but a senior FO on a heavy / captain on something lighter is a possibility I'm sure. (Not an official source / word on the matter however).
The bottom line though is this: If it is something you want to do - you should at-least try and do it - after-all, it is not what we do that we regret, it is what we did not do.