Probably 10 plus. When they first become interested in dating and have enough of a developed mind to know how they feel
My brother is 10 and he is already questioning being gay or bi. He is starting to figure things out for himself. He likes girls but he likes boys too.
That was also around the age I started thinking about being bi or lesbian (Back before I found out I was trans)
But anyone can find out at any age, of course. But that's just what I think makes the most sense