I know they taught it in Kindergarten. I think Dylan was doing it earlier, but not much. It wasn't on my radar of things to teach her, but we had a big number chart with numbers 1-100 and she discovered that numbers ending in 0 all lined up. She also figured out how to count by 5s this way.
I voted age five. It is a skill they learn in kindergarten in our school district. They also learn to count by one's to one hundred by counting how many days they have been in school each day and building the ability to count higher every school day.
She picked it up from starfall.com, but I found that it made it easier for her to learn counting to 100 by ones once she knew how to count by tens . . . since she could think to herself, "Ten, twenty, thirty, forty, etc" if she got to something like, "39" and couldn't remember where to go from there.
I'm sure my child could have memorized that at age 2, same as counting to 10. But he was approaching his 8th birthday when he really understood the concept, making all the connections on his own that you could figure out 20+30 by adding 2+3 and and converting it back to tens. But I'm not sure if he actually skip counts, as such... This was without any outside introduction or being taught it in any way. So I'm voting age 7 for a natural learner.