I don't think it's the age difference that's disturbing in itself. What makes this so squicky for me if that Sue apparently fell in love with Reed when she was freaking 12.
And a young twelve, at that. She's clearly being presented as a little girl, here, with a doll on the ground and a bandaged knee and everything. The idea that THIS is the beginning of her love for him is SO much more disturbing than it would be if, say, she'd started having these feelings for him when she was 18 and he was 24 (or whatever the age difference is).