You don't grow a lot after 16 years old but until then teens are usually shorter irl, depends on genetics too, so this "realistic" is quite subjective. Either way in a video game teens being shorter makes sense for the players, it's easier to visually distinguish from older lifestages and better gives a sense of growing while playing through generations. I don't know where you got that, but in TS4 I can only tell teens and YA apart by looking at the UI while hovering the mouse over them, just "paying attention" doesn't cut it. The current visual differences are too subtle, sometimes they even look like elders at first glance depending on what they wear or what hair color the game spawned them with.
All in all they are the same height as adults and young adults and elders because TS4 was meant to be MMO which probably wasn't even suppose to have lifestages at all. After restructuring Maxis had to cut corners where they could and rush an unfinished product to meet deadlines. Making teens a different height meant putting extra work, time and dime on the clothing assets and animations. It's not because they wanted to make them "realistic".
