I imagine them just about the exact same way.
There's such a huge age gap between toddlers to kids and kids to teenagers.
You have this toddler who's just learning to write or draw or whatever just got potty trained and learned to talk then blam! Now they're in school and doing homework.
Same thing for kids-teens. These little kids look no older than 10 years old, playing with doll houses and toy cars etc, then blam! Now they're old enough to be able to drive, and make out, have girlfriends/boyfriends and are the exact same size as adults.
I really liked it when teens were smaller than adults, because I could imagine them as the younger teens more easily, and then young adults in my head could be the "older" aged teens as well.