Voting is a minimal right, gun ownership isn’t. For one, owning a gun costs money. Voting doesn’t. It’s way better to tie voting to when someone’s labor or property can be taxed. The youngest age at which someone can work varies a lot by state, but it’s always < 18. It’s unjust for someone to be taxed but not be able to vote.

Also, FWIW, the drinking age is tied to how alcohol damages the brain. I’m not sure it makes a ton of sense though since making it illegal might contribute to more binge drinking anyway.