It’s an arbitrary cutoff point, like the voting age. There’s a drive in the UK to lower the voting age to 16, and in the Scottish independence the voting age was indeed 16. for obvious reasons, this is led by the left-leaning parties. It makes sense to just pick an age that denotes adulthood and stick to it. In the UK, you become an adult at 18, and can therefore do everything, from drinking to soldiering to voting. 17 for driving and 16 for marriage, supervised drinking and smoking, and joining the army, but not fighting. Basically, in my opinion there should be an age where you’re not subject to age limitation. In today’s juvenile world, where adolescence basically continues into the twenties, then 21 probably makes more sense than 18. But random age distinctions based on ad hoc reasoning is a little weird.
Either you’re an adult or not.