Quick look at Wikipedia will tell you:

Pedophilia (alternatively spelt paedophilia) is a psychiatric disorder in which an adult or older adolescent experiences a primary or exclusive sexual attraction to prepubescent children.

Meaning age <13.

Now in modern day Internet discussion people rarely bother with the actual meaning of the word and scream pedophilia whenever a <18 is involved, but those people are simply wrong.

There is also the legal age of consent, which gets defined by law and varies between states and countries, somewhere in the 14-18 range generally.

Also fun fact: Child marriage is still legal in a lot of the USA.