I agree with that statement in general. But to some extent there are exceptions. A number of variables may determing ones emotional age (upbringing, enviroment, brain chemistry, etc.). Some 18 yr olds are way more emotionally and mentally stable than some of their peers.
Lets not forget that in some other countries, people are often married at ages younger than 18. Out on their own and rasing families, doing fine.
Do I agree with you when you say?: "18yrold is still not emotionally self-sufficient, and isn't likely to make responsible life-choices."
Generally Yes.