I'm not entirely certain the addition of a pre-teen stage would really improve anything. The teens are themselves already not that different from adults, just based on what they're capable of doing, so unless the game went to lengths to distinguish the pre-teen stage from the teen and adult stages, it wouldn't really fix a problem, just create another one. As much as I'm in the camp that thinks '16 year old' when I think of a teenager, given the gigantic gap between seven and sixteen, I understand why there's demand for a noticeably younger teenager state, even if that's pre-teen. I just don't think it'd fix anything.
Personally, I use a height slider. I consider Sims being the same height in general as unrealistic and not for me. Unfortunately, a height slider wouldn't actually fix this problem, either. Essentially, if a built-in height slider was implemented, presumably the slider would work like any other slider, and different Sims would have randomized heights. You'd get short elders and tall teenagers, and it still wouldn't fix the visual difference problem. Or rather, there'd then be too much visual difference, and nobody would then be able to tell what anyone is.
Many people are not good at recognizing physical features. Some people are something called face blind, in which everyone essentially looks the same and the way they tell people apart is via something else: i.e. the kind of clothing they wear, or a certain necklace they favor, the color of or way they wear their hair. Making teens just a little bit shorter is probably the only solid solution to this problem. With the height slider, the only issue is with animations involving two different Sims not aligning properly. With a small enough difference between heights, the hug animations should be mostly fine, and teens can't romance adults anyway, so that height difference there on romantic interactions won't matter.
If people then want to make their shorter teens look like miniature adults, that's their business. If instead they decide to make their shorter teens look twelve, that's also their business. Everyone plays this game different and finds different things to be important to them, and that's okay.
