Twelve and sixteen isn't an enormous gap. But, personally, it would squick me out. I would seriously question the... sanity, intelligence, whatever... of a sixteen-year-old who sincerely wanted a sexual + romantic relationship with a twelve-year-old. Reading through this thread, though, makes me wonder if I'm a prude.
I can definitely imagine a little kid having a crush on an older kid, but the other way around just seems like... eh.
I'm the same way. When I was in my mid teens, pre teens were still children to me. My husband is 3.5 years younger than me, but if I'd known him when I was a sophomore in highschool (15-16), he would have been a little boy to me, and therefore completely uninteresting.
There's such a jump that happens when you hit puberty. Girls grow breasts and pubic hair and start their periods. Boys shoot up in height, and their voices get all scratchy and they get these huge, bobbing Adam's apples on their necks. Some get bristly right away, while it can take years for others to be able to grow any kind of beard. But girls vary in how quickly and how much their breasts grow too. And zits and body odor become banes of one's existence. Girls may mature mentally faster than boys on average, but a 16-17 year old boy is not generally in the same place
physically as a 12 year old girl (who is really not physically ready for sex or childbearing, even in our era of early menarche).
One big shift seems to be that crushes and infatuations shift from being puppy love to sexual in nature. I can't speak for all kids, but when I was 12, my fantasies about the boys I fell for entailed holding hands, a little smooching, and spending time together, not having sex. I wasn't sheltered growing up either. I knew what sex was, and my cousin and I even spent some rainy saturdays reading her parents dirty books (and they were really xxx, not just torrid romances), but the fascination was more akin to what I experienced when reading about bizarre medical procedures
From what I've heard, though, boys start fantasizing about actual penetrative sex earlier (possibly because it wouldn't hurt their bodies the way it would a physically immature girl's).
Again, some kids probably are different, and society is even more hyper sexualized today. Maybe girls start fantasizing about intercourse earlier too now. Maybe they did at some times in history as well.
I do agree that a lot of things that sound off putting in the abstract (when we're discussing them in forums) can work when they're presented in a way that's organic to the situation and characters.
That's pretty much the reaction I'm going for, which is promising to see.
For some reason, I find it less squicky when they're both girls, maybe because there just feels like less of a power differential there. Plus, the things two girls do together will likely not be painful to the younger one or result in either getting pregnant, which is a big issue I have with younger teens having sex, especially in a time or place where contraception isn't available.