The current thinking (I have packed) is go and see what happens. I am not stonkingly optimistic but on balance I suspect a DNF will be more exciting than sitting at home. And hey it might be OK (hmmmm).
There are some halves around which have a minimum age of 16 according to their websites - but in case they're not supposed to, I'll let you do the Googling W.I.A! The sort of thing you're looking for may be an "event" or "challenge" if you see what I mean - so not a UKA, TRA or ARC event.