Men who have sex with men (MSM) are disproportionately impacted by HIV and other sexually transmitted infections, with sexual behaviors acting as a primary predictor of disease acquisition. Predictors of engagement in specific sexual behaviors may act as key targets for preventive strategies. We hypothesized that time since first engaging in oral or anal sex with another man, or one's “gay age,” is associated with sexual behavior among MSM. We examined 5280 MSM aged 18 to 40 years who were recruited from social and sexual connection Web sites. We used modified Poisson regression to examine associations between gay age and 4 sexual behaviors (enema use, group sex, receptive anal intercourse, insertive anal intercourse). We used time-varying effect models to examine how the prevalence of these behaviors varies across gay age. In total, 76% of participants reported receptive anal intercourse in the past year, and 76% reported insertive anal intercourse. Group sex and enema use in the past year were reported by 39% and 36%, respectively. Modified Poisson and time-varying effect model analyses indicated that the prevalence of enema use, group sex, and insertive anal intercourse significantly increased with increasing gay age. Gay age may serve as an important marker of engagement in sexual behaviors associated with sexually transmitted infection/HIV acquisition among MSM.