Why are 'thick' people immature and childish?

No offence intended, but I have often noticed that stupid people are really immature and extremelly childish. I know my ex was he is 27 this year and acts like a spoilt child and his 25 year old sister, when I speak to her (I am 22) I feel like I am so much older and wiser, she says such stupid things and doesn't know how to take hints. I just wondered why this is!