There is copious evidence (much of it quoted by Kinsey) of children masturbating at all ages from a few months old onwards.

Kinsey mentions one record of a seven-month-old infant and records of five infants under one year of age who were observed in masturbation. Some of the cited evidence includes Townsend (five cases under one year of age); Talmey (one case of nine months); Spitz (rocking and genital play observed in over half of 248 children under one year of age), etc.