He isn't dense, he knows the girls like him.
Its just that he is too focused on defeating the person who killed his sister first.
Until thats dealt with, he feels it wouldn't be right to start a relationship. Because he wouldn't be able to focus on it and the girl he choose might meet the same fate as his sister if he did.
You gotta pay more attention to the series, since everything is explained for you.
Would you start a relationship with a girl if you knew someone planned to ruin your life or kill your gf when you were away or not paying attention?
Of course not, nobody would be stupid enough to do that.
Its matter of principle with Tooru, he has to deal with his personal problems first before he can get anywhere in life.
Just because she looks young doesn't mean she isn't the same age as the other characters
You can't judge someone just by their appearance alone.
Also you do realize that none of the characters ages are actually given in this series, right?
So your opinion doesn't mean much in that case.
He likes Julie, and possibly has have some interest in miyabi.