I think it’s a problem they don’t get vote in the first place. If 90-year-olds can, so can 15-year olds. My money is on the average 14-year-old to make better decisions (40% of 90-year-olds have dementia, so that’s a safe bet).
I’d say the age of criminal responsibility - between 12 and 15 in most of Europe - seems like a reasonable cut-off. If you’re too immature to vote on a law, you’re too young to go to prison for breaking it. We could however leave passive suffrage to the actual age of maturity (and full criminal responsibility).