On sexual orientation? I don't see why there should be a bottom limit, because sex does not need to be (and should not need to be) explicitly discussed vis-a-vis romantic relationships. The youngest school aged kids know about mommies and daddies/husbands and wives. So it's as easy as saying "Most kids have a mommy and a daddy, but a few kids have two mommies, and a few kids have two daddies." I do not think such a revelation would necessarily cause little kids to fall to the ground rolling and frothing in seizures.
On sexuality in general and sex education, I would mirror
@tacomancer. Sixth Grade is perfectly acceptable.
As far as "gender identity"? I have no idea when that would be appropriate, because such discussions would probably serve to draw attention to children who identify as transgender. So I will demur on that.