Because he looks like he's 7 and not almost 13. The twins are like doorknob height. When I was almost 13, I was taller than many 15 year old girls and dating them would have been perfectly fine.
shareThey're better off as great friends in the long run. The age thing I don't really find much of a concern. It's the fact that Dipper is a really awkward kid and an insistent romantic relationship with Wendy might threaten the strong friendship he already has with her.
The reason why a relationship like Ferb and Vanessa worked was because Ferb, although a kid, seemed wise beyond his years and really wasn't in a rush to find a girl. He'll be a kid first and he'll find a gal down the road. Although he had a crush on Vanessa, he figured he wouldn't have a shot, but always left the door open if the possibility for a relationship in the future was there. Lo and behold, he's a happy man.
I view Wendy giving her hat to Dipper as a sign that he'll one day grow up and rid his awkwardness (or most of it anyways) to mature and be the coolest guy of Gravity Falls.
Plus, Dipper has Pacifica, who I think (based on AH calling her an insecure girl), will have Helga Pataki-type tendencies to deny any potential crush she might have on Dipper, while also being out of sight and watching him from afar (yup, I ship Dipcifica.)
There's definitely no arthritis in this group!