Your licence is valid for 7 years from date of issue by licensing authority regardless of whether or not you use it (I think). Your ratings however are a different story and must be revalidated at specific intervals (annually for the IR), or renewed within a longer period of time (5 years for the IR), otherwise they lapse and you have to resit the initial test again with an examiner from the authority rather than a regular FI approved to conduct exams on their behalf. I'll let you guess who is more scary.
Pointless having a licence without ratings.
On the subject of age, whilst you need to be 18 to commence training for the ATPL, you are unlikely to be finished training before you are 19 (most integrated courses take approx 14 months or so), and the hours required for the issuing of a full ATPL are unlikely to be earned in much less that 2 years operational flying (CAA limit of 800 hrs annually I believe), so it is unlikely in the extreme that anybody under 21 years of age will be in the position of actually meeting the requirements for a full ATPL. The lack of a full ATPL does not preclude you from working as a First Officer, only as a Commander on multi-crew aircraft.