my brother! i dont get it

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he is a year older than me yet i feel as though he is my baby brother, he is not the sharpest tool in the shed nor has he the mental capacity to realise that he is nearly 18 and still acts like a 12 yr old! i dont mean to say that i am oh so above his level but i just dont get him, does any1 have any problems like this or any idea why he is like this?