Yup, the moment he turns 18, he's automatically legal to buy and be sold an
IF. He's also legal to buy and be sold a
RIF as long as the purpose is for use at an insured airsoft site (and one way of demonstrating that is site membership
or a UKARA number). If the site won't even start his membership qualification until he turns 18, then in practice the minimum age for getting a membership or UKARA defence rises to 18.15384615384615384615384615384615 (I discount leap years here for the sake of simplicity).
If he can keep a record of the game dates he can ask them once he's done the three.
Its supposed to be with a 3 month period
That may be a specific site's policy regarding membership, but it's not part of the UKARA scheme - that's a minimum of 3 games over a minimum of 8 weeks. There's no maximum mentioned in the scheme itself (
http://ukara.org.uk/ ), and indeed it doesn't even mention expiry or renewal although there seems to be a policy of expiring after a year.
Although even at that, I wouldn't be convinced by the scheme runners' competency to do any such thing, or that retailers or Border Farce actually bother checking anyway as long as the number looks legit.
Try it and see.