As you know, I share your commitment to not making adult-child sexual activity more acceptable or legalizing it.
But these two issues you have raised (aging out and STIs) seem to me frankly a bit bizarre. Are you here trying to actually learn anything or change anyone's mind? Or is it just, "I thought of it, so I'll post it"?
First, take off the table cases of adults doing things with struggling or screaming children. No one's going to defend that. The guy who defends child marriage is a lone opinion too. What people here at GC will defend is apparently loving relationships with apparently enthusiastic girls -- in a society where that was legal and accepted.
Children can get STIs from other children who get them from older children -- relationships that with "Romeo and Juliet" exceptions might all be legal. On the other hand, exclusive pedophiles are extremely unlikely to have STIs at all because we typically have very few adult partners. In my reading, it is also very rare for anyone here to defend penetrative intercourse with young girls.
If people here heeded your advice, they might say, "Well, I'll be sure to have myself tested for STIs before I have any sex with girls." Surely that's not what you're aiming for? And don't you care for adults? Adults too are at risk of STIs, so your argument would suggest that adults shouldn't have sex either.
If they heed your advice about aging out, they might well check and reassure themselves that they would never dump an underage girl.
Your arguments could actually reinforce a pedophile's conviction that adult-child sex might be fine.
What will we hear next from you? Girls could be killed in car crashes if they are driving around with pedophiles. Too much time spent with a pedophile could get in the way of a girl getting her homework done. A girl might catch a cold from hanging out with a pedophile.
The key issue is potential emotional harm to the girl. That's THE number one reason, and if you were trying to convince people here (at least the newbies or some lurkers -- you'll never convince the long-time frequent posters), you'd do well to focus on the key reasons. I've addressed this whole pro-legalization in a series of essays/blog posts here: http://celibatepedos.blogspot.com/2017/03/index-considering-pro-legalization-pro.html
When anti-pedophile activists make bad arguments, it reinforces pro-legalization pedophiles in their views. So far you've been making bad arguments.