You got me wrong too :)

The only reason you don't consider an 18 year old an adult is that we don't give a person adult duties and responsibilities until 18 - so it takes till 25 till they are a "real" adult.

I say, give them those duties and responsibilities at 15. Then they will be an adult earlier.

If we did like you say, and wait till they are 25, then all that will happen is that you will not consider 25 years old adults, and you'll want to wait till 30.