If you are in the UK you will be able to get the CPL/frozen ATPL before you are 21. Then when you have 1500 hrs you can upgrade to your ATPL, however these hours have to conform to certain criteria. You will be doing v well to have the hours by your 21st birthday. It's no biggie anyway unless you are planning to be a captain before you're 21 as you can fly as a co any jet-transport a/c with a CPL.
That's certainly how i remember it used to work - it may have changed with JAR-OPS