granted, I'm a short adult, so I don't mind the teens being tall. I used to tease the kids at our church that I wouldn't believe they were teenagers until they could look me in the eye. Teens aren't all short, and I've known some middle schoolers that were taller than me.
That said, it's all about the Animation budget. Dynamic rigging was (I think) still relatively new (read expensive) to code when TS4 was in development. It's now less so and hopefully, it can be incorporated in to TS5