On the episode "Amok Time" Spock explains the Pon Farr happens every seven years. Does that mean they "Pon Farr" when they're seven and fourteen!?
No. It seems to start sometime after the onset of puberty.
In that scene, Spock is in his mid to late teens and is experiencing what appears to be his first Pon Farr based on Saavik's comment "so, it has come"
The onset of Pon Farr for the regenerated Spock in Star Trek III occurred when the character was played by Stephen Manley, who was credited as "Spock, Age 17." So, for Spock, it was definitely 17, although it probably varies a bit from individual to individual.