That is irrelevant. If a 5-year old wins some competition, they will pay taxes on that money. That doesn't mean we should give 5-year olds the vote, does it? Also, plenty of non-tax related laws apply to children - why should they not get to vote on Education or Healthcare, for example?

Also, while some 16-year olds work and pay taxes, the majority do not. So a blanket right to vote is not justified even by this strange criterion.