You have to be 21 to have an ATPL issued. The intergrated course gets you a CPL, which has an age requirement of 18. To convert a CPL to an ATPL you need 1500 hours, a large number of which need to be in multi-crew aircraft, at night and/or IFR, etc. I would be rather envious of anybody who can do that before they're 21. Of course once you have unfrozen your ATPL you would only use it once somebody lets you captain one of their multi-crew aircraft. Another enviable feat for a 21 year old. Check lasors for details, if you aren't already confused.
Regards
Stoney