Wellllll~...

Let's just say, touching someone's head is considered very rude, incredibly so if you're not family.

Head-patting among the younger generations (under the influence of foreign *cough*'murican*japanese*cough* culture) is slowly becoming the norm, though. Just don't do it in front of oldfags.

Also, you risk being labeled a pedophile.

(Speaking as a Thai)