There is no minimum age to sit your atpl, well there is LEGALLY, and that is 17.
You can demonstrate the same amount of determination/ maturity if your 17 or your 30...it makes no odds, it just depends on how much of a gambler/determined you are.
If you are 100% sure your want to be a pilot, then start it as soon as you can, because now is the time where jobs are rife.
There are too many people that delay starting their training and find themselves 3 to 4 years behind the drag curve....and that could be the difference of £200000 if you find yourself with the right employer.
I'm not going to blabber on about the choices i've made, but suffice to say, I started young and have no regrets. Its all about maximising your investment. SO
why spend money on a degree that your not going to follow up?
Bite the bullet, follow your dream and the sooner you start, the sooner your debt will be paid off!
good luck.
randomair