The represented age of teens should be brought down to about an age of 13-15 to fill this lifestage gap. The decision to make teens an apparent age of 18 or so has good intentions, such as giving them more things to do, but more isn't always better, because that made teens too much like adults. This would bring more distinction between child, teen, and young adult, having the characteristics and activities that OP wants, while not making the same mistake in the opposite direction, and having preteens be just children with a few other stuff. By changing instead of adding, it also avoids the problem of having too many life stages.