I think voting should be restricted to legal adults. As should serving in the armed forces, marriage, driving and anything that involves someone needing to enter into a contract.
There may be an argument for reducing the age of legal adulthood to 16, I don’t have views about that, but let’s keep voting for adults.
I would prefer to see long-term legal residents given the right to vote, even if they’re not UK nationals. You lose the right to vote if you don’t live in the UK for 15 years; you should gain the right to vote if you’re here legally for that period of time.