Jason was born in 1946. He "drowned" in 1957. F13 Part 2 takes place in 1982. Jason is 36 years old at the time of that movie. The legend goes that he drowned, but his body was never recovered which would hint that he did infact survive the drowning.
I disagree. I always found the concept of a copycat killer of the original killer to be the replacement for the next sequel to be quite lame. I'm glad they changed their minds about it and made that part of the movie another dream sequence.
Spoiler (Highlight to Read):
Jason's body is taken to the Wessex County Morgue, from which he escapes. After returning to Crystal Lake and killing several people, Jason is killed by 12 year-old Tommy Jarvis. This unbalances Tommy, who spends the next four years in mental institutions. Jason's body is buried in secret at Eternal Peace Cemetery. (Friday the 13th: The Final Chapter </films/friday4.html>)
1985
In an attempt to disassociate itself from Jason Voorhees, Crystal Lake changes its name to Forest Green.
1987
Tommy, now 15, is moved from an institution to a halfway house for troubled teenagers. The son of ambulance attendant Roy Burns is killed by one of the kids and Roy, masquerading as Jason, methodically kills the others. Tommy is forced to kill Roy, further affecting his mental state. Tommy attacks Pam, one of the survivors, but is subdued and returned to an institution. (Friday the 13th Part V </films/friday5.html>)
1988
May 12-13 (Friday) - Tommy, now 16, is released from the mental institution. When he digs up Jason's grave, intending to cremate him, Jason is reanimated by a bolt of lightning. Jason kills his way toward Camp Forest Green, which is once again attempting to open its doors to campers. Tommy drowns Jason in Lake Forest Green (Crystal Lake)
and I would assume he rode off into the sunset with his girlfriend satisfied that he thought he ended the terror.