Charlie's pre-K teacher said that a lot of kids skip the number 15. She said the reason is that you say it "fif-teen" instead of "five-teen." So kids don't relate it to the number five... unlike say "four-teen" or "six-teen."
It's a great theory, although it doesn't really explain how most kids don't have trouble with "eleven," "twelve" or "thirteen"!
Can your LO count to 20 without skipping numbers?