Those numbers aren't absolute which is the entire point of the AoC being set at 18. There are some who will have started and finished puberty before they are 10. They will be outliers but it happens. Conversely late development can mean even those who are 18 and over may not yet have finished going through puberty.

So laws attempt to come up with a number that works for almost all cases however life being the crazy box of frogs that it is "finds a way" to cause trouble.