What age do you consider someone grown?

I read that the brain is still developing and maturing even in the 20's and it seems like after 30's also.The Frontal lobe is that part that controls judgement and risk taking and drinking doing drugs before its mature can affect and alter your brain development.This 58 year old woman who is the mother of Ray J and Brandy told Ray J who is 29 that he would be more mature at 35 or 40 to get married and be in a serious relationship and he is very immature he even made a tape.I think the age of adulthood personally should be 40 cause the brain would be on an adult level by then and all adult rights or realistically maybe 21.