That's not a universal truth.
I was basically 2 different persons at 18 and 25. Now at 36 I can even say I'm completely removed from the young "Dancer" I used to be at 18, and I'm not talking physically.
People have different maturity developments, what applies to one does not apply to another.
Naturally it can go all over the place and people can also develop terrible character traits and worsen with age, but saying that if you are a mental midget at 18 means you will be one at 25 is just not true for everyone.