According to the BBC website checker, in my area of SW London, the current cases in ages 0-59 far exceed those of ages 60+.
Which would seem to indicate that the vaccine is having a major effect, so ?.
The one young person (21) I know who’s had it fairly recently, a student niece, had it only very mildly. TBH I was surprised she didn’t get it sooner, since she was racketing around Europe with a lot of friends all through the Christmas and Easter holidays. And none of them paying much attention, I dare say, to social distancing.
Should add that she was unable to go home (to Singapore) for either period because the compulsory hotel quarantine would have used up virtually all her time.