"When did you know?"
Stop asking people who don't identify as straight that question.
I hear straight people spew this question to people whom self-identify as gay, bisexual, asexual, or anything not straight all the time. When did they know they were attracted to the same sex instead of the opposite sex? Or when they weren't sexually attracted to anyone at all? Well, my question is: Why do you have such a dying urge to know?
When did you find out that you were straight? Did you just pop out of the womb thinking, "I can't wait to marry and procreate with someone of the opposite sex?" Is that when you knew? Doubtful. A person who doesn't identify as heterosexual didn't necessarily have to have some revelation to discover that person's sexual orientation.
How many people ask you when you found out you were straight? Presumably, none. If you haven't struggled with discovering your sexuality and you've identified as straight for as long as you can remember, you can assume that some people have had the same experience with their sexuality, even if they aren't straight. Similarly, if someone has struggled a lot with their sexual orientation, and is currently struggling, questions about when they came to realize their sexual orientation is unnecessary.
If a person that doesn't identify as straight wants to tell you when he/she/they, (supposing that the person came to know his/her/their sexual identity at a particular time) then that is that person's choice. But if the person doesn't want to tell you, just hold your tongue.
The way that heteronormativity permeates our society has tricked individuals into thinking that everyone is born heterosexual; everyone comes into this world with the desire to have some heterosexual sex! Wrong. Those assumptions are simply bigoted to have and create an unsafe space for those who do not identify as straight. No one wants to feel like they were born the "wrong way," so entering the world with this theory that you identify as the "normal" sexual orientation isn't cool. This is really important. Because not being straight is already hard enough in our intolerant society. If you know a person who doesn't identify as straight, there's a possibility that that person has known their sexual orientation for as long as you have.
The point is: Sexual orientation varies amongst almost everyone. It's fine if you've known that you are straight for your entire life. But it's also fine for people who aren't straight to know that for just as long. Or to have just realized it the other day. Chances are no one ever pokes at you to reveal the life-changing moment that forced you to realize your straightness. So, don't put that kind of pressure on non-straight people. They might not have a story to tell, they might have a Nobel Prize winning story to tell. Either way, it's not your place to ask, and you don't need to know.