I was 7. A proctologist used a speculum on me and I liked how it felt. Just kind of snowballed from there... just like when a boy jerks off for the first time, he just finds more ways to do it and more frequently.
I am sure it goes back to childhood. Grew up when rectal thermometers were common for kids, so there was always some fascination with the subject. Can recall exploring back there during bath time as a child. Just stuck with me over the years.