Purpose: Sexual education in adolescence may represent the only formal sexual information individuals ever receive. It is unclear whether this early educational experience is sufficient to promote lifelong sexual health literacy. The purpose of this paper is to examine the influence of the timing and source of sexual knowledge on current safe sex knowledge and risky sexual behaviours among middle-aged and older adults in the USA. Design/methodology/approach: Participants (n = 410, "mean age" = 53.9, 50.7 per cent female) reported whether and when they received sexual knowledge from various sources. They were asked about their current safe sex knowledge and their lifetime sexual risk behaviours. Findings: Most of the participants (61.5 per cent) received formal sexual education in adolescence and 20.2 per cent reported formal sexual education post-adolescence. Across the life span, friends were the most common source of sexual information. The sample scored in the upper mid-range on the scale indexing safe sex knowledge (M = 6.69, SD = 1.64, range = 0-8). Participants reported engaging in an average of approximately four (out of 16) risky sexual behaviours across their lifetime. Those with formal sex education in adolescence scored significantly higher on safe sex knowledge. However, they also engaged in more risky sex behaviours. Originality/value: This study is among the first to situate the normative, formal sexual education experience of adolescence within a life span context that not only accounts for time, but also multiple sources of influence. It would appear that there are more things to learn about the long-term influence of sexual education programmes during the formative years by studying adult sexual health and knowledge.