[Mysterise]

It's the age where the majority of legal systems draw the line on considering a teenager to be an adult, so it isn't unreasonable.

[josefx]

This is the age where the last legal distinction is removed, there tend to be a lot of age related regulations that change over the years before that.

18 is pretty much only reasonable when it directly aligns with a legal requirement. Otherwise it is not the most reasonable, but the most lazy choice.