OK, go back and read the Violent Crime Reduction Act 2006, I'm not sure which version you've been reading but you've clearly missed a few bits out.
The only things specifically controlled by the act are sale, import and manufacture.
In order to sell a RIF to someone you need to avail yourself of a defence against the VCRA, the person buying doesn't need a defence (they simply enable the seller to prove their defence), they're not committing a crime. Usually this is done by the seller checking the buyer's bona fides and satisfying themselves that the purchase is indeed for one of the permitted activities, UKARA is just a method of making that easier, nothing else.
Import of a RIF can be done by anyone who can prove materially to customs and excise (more specifically the UKBA) that they are importing it for the purpose of one of the permitted activities. At this time the only method they accept is a site membership recorded on the UKARA database or membership of a recognised reenactment organisation (e.g. AFRA).
Manufacture of a RIF is not controlled by age in any way (read the legislation, it just isn't), not only that, the conditions that need to be met in order to qualify for any of the specific defences (as detailed in Home Office circular 031 / 2007) are not quantified in the act itself. Personally, I would not manufacture a RIF unless I could materially prove that I am a regular skirmisher, be that with photographic evidence stretching back several years or by a phonecall to one of my local site's owners who would verify I am legit.
The statement however that an under 18 cannot under any circumstances own a RIF is completely false, manufacture is an entirely legal way for an under 18 player to get themselves a RIF providing they are confident that their defence against prosecution would hold up under the kind of close scrutiny the CPS would put it under.