What year did punk rock begin?

There is no one answer to this, as artists such as The Stooges, Patti Smith, MC5, and The New York Dolls incorporated elements of punk into their music, but are generally regarded as "proto punk," meaning that they influenced the punk genre, but came before it and are not a part of it.

The Ramones, formed in 1974, are widely regarded as the first American punk band.
The Sex Pistols, formed in 1975, are considered the first British punk band.