It will bite them in the bum.
I incline to giving a pass to adults on their teenage fuckery, to a lesser extent early 20s fuckery depending on the type of fuckery. I’ve also seen first hand the postive impact of rehabilitated extremists (and criminals in general) in countering those views amongst young men/teens susceptible to the same messages. If he is genuine and successfully doing preventive work for eight years I can see why an organisation or public body might employ his services.
But - with that level of offences, continuing into his 20s its too soon to be standing even as a lowly councillor. If he were ten years older with nearly 20 years of good prevention work behind him then maybe.
I do have differing expectations for low office vs high office.