Okay, for JAA ATPL you need to hold a CPL+IR and have completed the ATPL exams, this is known as a "frozen" ATPL. When you have 1500 hours, of which 500 must be multi crew aircraft, you "unfreeze" your licence and gain a full ATPL.
The ATPL theory credit is renewed by your instrument rating. You have seven years from the date of
your last IR renewal. It doesn't matter if its a Single pilot IR or a Multi Crew type IR. To hold a CPL you must be 18 years or older, for the full ATPL you must be 21 or over.
Hope this answers your questions
Best regards
MP
P.S.
I was under the impression you could be in possession of a fATPL at the age of 18? entitleing you to your first job as a FO, but you have to be over 21 and have the flying requirements 1500+ hrs etc. in order to have the licence 'unfrozen'.
That's correct.