If my boyfriend says he's not going anywhere and then he said he just 20yr old and then he said i don't understand him what does he mean by that?

He means that you have to trust him,he will not going to leave you with another girl even though he is just 20years old but you have to trust him .don't build a fences around him you have to understand him.If he is with a friends or anywhere   do trust him ,he will alway be there for you.
I know that us women have a very sensitive feelings.

It sounds like he is being evasive however one would have to know more about the conversation to accertain that. It sounds like you have give a response to an on going agrument and thus one would need to know what was said leading up to this comment to make any determination as to its meaning.