You don't have to fill it in every year, but you do have to update it if it has changed. Young people move around the country a lot, so could be on the electoral roll in a place in they're no longer living, and hence don't have access to their vote.I thought so too tbh but the amount of things i have seen in the UK recently encouraging teens to get on the register makes me wonder
just mentioned this discussion to my flatmate and he said we get a thing from the Council every yr if yo don't actually fill it in every time you get wiped off the register. You will get fined if you don't and it's a legal requirement .
Perhaps you just have to check you are on there if you are a teen/new to it/young person rather
Reregistering is one of the first things I do, personally, whem I move, but then I can see how it slips.