Apparently you're not meant to be able to tickle yourself. It often comes up in things like trivia quizzes and the like that the only place you can tickle yourself is the roof of your mouth with your tongue but you can't tickle yourself in other places.
However I've always been able to tickle myself. My feet are the most ticklish but I can tickle myself under my ribs and armpits too. It tickles as much as when someone else tickles me.
I did ask Google this but all it turned up was pages on Schizophrenia and about how being able to tickle yourself was a sign of schizophrenia and schizophrenics can tickle themselves but other people can't, etc
. Which freaked me out.
I'm pretty sure I'm not schizophrenic...however I can't be the only non-schizophrenic person in the world who can tickle themselves!