he became a vampire in 1935. (you better not steal my monkey man!)
1935
Emmett Cullen was born in 1915 and his vampire transformation was in 1935
the year of 1918
It doesn't say in the books.
Around 1920
After being mauled by a bear in 1935, Emmett was rescued by Rosalie Hale, and given a second chance at life after being transformed into a vampire by Carlisle Cullen at her request. Rosalie and Emmett were rarely parted after this, being that they were true soul mates.
Edward Cullen became a vampire in 1918 at the age of 17.
1935
Carlisle Cullen changed Emmett Cullen in 1935. (Rosalie saved him, he was mauled by a bear.)
The members of the Cullen family that remain at Forks High School are Edward, Alice, and Jasper Cullen. Therefore, by process of elimination; Emmett and Rosalie have already graduated a year before Edward Cullen and Bella Swan.
Jasper Hale is the oldest of the Cullen adopted children, if you mean by the year they became a vampire. Next in line is Edward, followed by Alice, Rosalie and finally Emmett. In age when they became a vampire, Emmett and Jasper and Alice were all 20 years old, Rosalie was 18 and Edward was 17.
She was changed into a vampire in 1920