First off ive had these same issues, (and please excuse my spelling im deliexic) i have had ocd for around 6 years now and have no idea why it came on it could be to use of cannabis or the death of my father both could have had an inpact,
By the way you discribed it it sound very very clearly ocd, pocd.
Humans are can can be attracted to other humans of all ages and sex, pederfiles are very muched talked about in this day and age you hear about it in the news papers people maby down your street who knows but it is widely talked about. And this can play into your mind.
You may have felt aroused by many many resaons heres a few
Its wrong and tabbo you shouldnt be thinking about it so it makes you want to.
A human see's visually sexual organs of someone of any age and these thoughts can be very intrusive but we can become arounsed.
It is in your head so much that you make belive that you are aroused and become arounsed becuase of that. I could go on and on...
Its sound perfectly normal what you are talking about thinking is just thinking its not doing when you start doing then you have a major problem, but if you over think about these thing and get very gitliy and have alot of shame because of it the human mind can trick us + our ocd in thinking we would want to betray the thoughts were having.
Please dont get worryed about this i had them too and im such a loveing caring person and i see the real true beauity in life and i'd never want to do theses things but my problem is that i hated pedofiles which a passion or men who hit women or anything i found just horrible my ocd attacked me on and made me think i wanted to do these thing,
You seem like a very sweet girl dont worry your self your human as ive said 25 time allready

I hope ive helped x