I very much question the players who can't distinguish the teens from young adult and adult sims in-game. Are they not very observant? The teens in sims 4 are quite easy for me to distinguish in-game. Perhaps it is because I know they have a more narrow shoulder build and female sims have smaller busts (unless you make them differently in CAS).
I usually make my own pre-teen by viewing attention to the age bar. A child has 13 days on normal lifespan. I dress them to look like young kids at first and by day 6 or 7 days of their lifespan, I dress them to look like an older kid/preteen. Once the sim ages to a teen, they have 21 days on normal lifespan. I do the same as I did for the child lifestage and dress them to look very young teen for about 9 to 10 days. I then dress them to look like an older teen after that. You can adjust the timing if you play on short, long or even aging off.
I think introducing another lifestage at this point in a 10+ year old game is only going to introduce more severe bug issues. We already have very buggy infants and toddlers, so that is why I view it as a bad thing. Also think about the amount of money it would take to make a whole new lifestage do all the different interactions needed.