I also disagree with the dehumanization argument. Yes, the concrete age limits are a bit arbitrary, and actual maturity depends on the individual and is a spectrum, but regulations necessarily have to come with some level of pigeonholing. The fact that you may have to show your ID when buying alcohol isn’t dehumanizing.
I'm sure teenagers feel the same way.
"Children" needs further qualification as well. Anglosphere calls anything under 18 years old a "child", which completely disregards adolescence where major brain development takes place. Some five year old generally isn't capable of deciding things, but teenagers absolutely are.