"3 out of 4 syphilis infections are acquired between 15 and 30 years of age."
This infographic breaks down the percentage of new cases of syphilis each year by age. Most who acquire the disease do so between the ages of 15 and 30.
"How syphilis was discovered in 1000 consecutive cases referred to the syphilis clinic by other divisions of the Johns Hopkins Dispensary : Data from Albert Keidel, M.D., Archives of Dermotology and Syphilology, Vol. 25, p. 470, 1932."