My interest definitely started before puberty. I think I was about 6 or 7 when I started to get curious about other boys/men getting injections in their buttocks. I used to wonder what they looked like when they got the injections and how they reacted to getting them. I also used to love it when I saw this in a movie or televsion show - wish it would be shown more often.
As well, when I was a little older, I used to ask my cousins if they ever had to get injections in their butt, and a few of them did say they had. I tried to get as much info about their experience about it, and I have to say, I usually had an erection when they were telling me about it.
I also remember a couple of them mentioned how some friends of theirs either had to get their temperature taken rectally or had to get suppositories, which also piqued my interest.
Am glad to see that I am not alone in developing this fetish early.