What is that based on, though? Why a single age for everything, when it might make sense to have it more "targeted". For example, wouldn't it make sense to allow voting in local elections, where things are usually simpler and cause and effect clearer, at a younger age?
Similarly, why tie drinking regulations, which are based on physiology, to voting age, which has nothing to do with it? You may say it's because if the person is mature enough to vote they can decide themselves, but there is a huge amount of things I'm not allowed to buy or consume even if I'm allowed to vote, so that argument doesn't hold (unless you advocate 100% liberalization of everything).
Having just a single age limit just makes it all seem very arbitrary, which it shouldn't be.
Why?