These are the RLMS data (round 19, 2010).
The age at different personal events is naturally different, median age at menarche is twelve, median age of the sexual debut is seventeen.
Age at which a half of those who had experienced periods, became biologically ready for conception and started their sexual life is nineteen.
These ages are important to consider the beginning of a course of sexuality education: when a school should start it? Imho: fourteen is the latest, or about the seventh grade.
The next important finding is fifteen percent of those who did not start their sexual life by age 35. It is very close to WHO estimates of infertility.