Can anyone explain why a 30 year old would be in a relationship with a 19 year old?

My friend’s cousin is 30 and still goes out clubbing, likes to do drugs occasionally (possibly has a problem) and loves to drink, his current girlfriend is 18 and is quite an attention seeker, quite rude when she starts to dislike some one and always trying to seek approval…

I would like to know what people think of such a relationship between a 30 year old and a 18, is it normal? is he still trying so hard to fit with the younger crowd? Please explain your thoughts :)