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.
Vorik on Star Trek Voyager looks as though he is about 21 to 28 years old. They tell us that Volcans go through the pon farr every 7 years of their adult life. The pon farr should start at the end of puberty. I would say the first pon farr would start at age 21.