How old was japer when he became a vampirein twilight?

Answer 1

Jasper was Aged 20.

Here's a little more detail on the subject:

"After becoming a major in the Confederate Army, Jasper was beset upon by the vampire Maria, who proceeded to turn him. After the transformation, Jasper aided Maria in her creation of a newborn vampire army which successfully conquered the feeding grounds of Texas. However, Jasper became disillusioned with this violent life and eventually became a nomad. Fortunately, he was eventually found by Alice, the love of his life and with her began a life with the peaceful coven of the Cullen Family."

Taken From http://twilightsaga.wikia.com/wiki/Jasper_Hale