Serious answer incoming:
I am just going to assume that you are in middle or high school. Ask your self this, "Do I or my peers have sex?" if you answered yes, then ponder this; you probably thought/knew that you/your peers had sex one year ago when you were in a grade lower. Now think about your peers in the grade below you, does it seem weird that they may/are having sex? When I was in middle school, as early as 6th grade, I thought I should be having sex because I thought my peers were and I thought I was old and mature. Now when I look at 6th graders I think that it would be fucked up if they were having sex. It kind of changes when you hit junior year of high school because you can drive and have a job and slightly more freedoms so you think you are a lot more mature, but probably doesn't change that much if you were a senior looking back at juniors.
Now imagine you are in college, lets just say junior year, and you are asking yourself the same question and you are looking on these comparatively young 16-17 year olds having sex. It probably seems almost as weird as a 6th grader having sex while looking back as a high school junior.
TL/DR: Probably like 8 years old, go and buy some mini-jimmies after you read what I wrote and reflect on it regarding your own ideals and beliefs
