Honestly, after the addition of infants, the thought of yet another life stage younger than teen makes me tired, lol.
I've always seen the fundamental problem as being the fact that teens look too much like YAs and have the same skills and gameplay as YAs. If they had made teens shorter and more baby-faced than YAs, and if they had given them some unique gameplay and skill-building parameters, we wouldn't need to ask for yet another youth life stage that feels less like the adult life stages. But you know they're not going to go back and overhaul teens now. And I think we're stuck with the youth life stages we have now; they're not going to add another.
IMO the biggest leap between life stages remaining now is between adult and elder. Adults look like they go from being 40 to being 80; it's ridiculous. They missed the opportunity to add a true middle-age life stage with Growing Together. Instead, they gave mid-life crises to sims who are apparently still young enough to have babies. So silly. Setting aside the fact that I'm not buying any more packs because of lack of QA, chronic bug issues and the in-game shopping cart, the longer I go without buying Growing Together, the less I think I need it. I feel like it would just add a lot of illogical mess to my game due to weird or lazy decisions like that one.