At what age difference is someone considered a pedophile by law?

There is no such thing as a "pedophile" legally. The legal terms for sexual activity involving underage minors include "child sexual abuse", "statutory rape", "sexual abuse of a minor" and "sexual assault".

According to the American Psychiatric Association, a pedophile is an adult or an adolescent aged 16 or older who is primarily attracted to prepubescent children (generally aged 13 and under). In order for one to be classified as a pedophile, there must be a 5 year age difference between the pedophile and his/her preferred partners.

If you are talking about the age difference in which someone is considered off limits to have relations with, that will depend on the jurisdiction you are in. In many countries/states, there are close-in-age exemptions that exclude punishments for sex with underage minors if the older partner is close in age. For example, in Canada, the age of consent is 16, but those aged 14-15 may have relations with partners no older than four years than them. There are no close in age exemptions in jurisdictions such as California and the UK.