In the study involving 5,000 individuals, 48 percent of first-born women and 37 percent of first-born men reported discussing sex with a parent at age 14, as opposed to 40 percent of middle-born women and 29 percent of middle-born men.
"In addition to seeing differences in sex education according to birth order, we also found clear differences between the sexes; across all birth order categories, men consistently reported lower parental involvement in sex education than women," said lead researcher Lotte Elton from London School of Hygiene and Tropical Medicine (LSHTM).
The researchers used data from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), the largest scientific studies of sexual health and lifestyles in Britain.
The studies have been carried out every 10 years since 1990, and have involved interviews with more than 45,000 people to date.
Taking a sub-sample of Natsal-3 participants -- 5,000 individuals aged 17 to 29 who were either first-born, middle-born or last-born -- the team analyzed responses to questions about the involvement of parents or siblings in sex education and early sexual experiences.
Later-born men were also less likely to report learning about sex from their mothers, while last-born women were less likely than first-born women to report a parent as the main source of sex education.
Although there were differences by birth order in parental involvement in sex education, there did not appear to be an association between birth order and early sexual experiences, although middle-born men were at increased odds of being under 16 when they first had sex, the study said.
(With inputs from agencies.)