I'm a developmental psychologist and I study child and youth development - I'm supposed to be writing a chapter on late childhood right now...
So here's my thoughts.
After infants were introduced, I started playing toddler differently.
Newborns are 0-1 month old.
Infants are 2-18 months old.
Toddlers are 2-6 years old.
Children are 7-12 years old.
Teens are 16-19 years old.
Young Adults are 20-35 years old.
Middle Adults are 35-60 years old.
Elders are 60+ years old.
At least from my perception of what they can do in game. I used to play toddlers as just 1-3 year olds, but they genuinely fill the young childhood gap for me. Children are filling the preteen gap for me.
But what's missing? Clearly young adolescents. Our teens are too similar to young adults. Would just a height slider work? The problem with that is young teens don't tend to have as much breast development, muscle development, vocal changes, shoulder and hip width that are represented in current teens. It's not just height.
In terms of game play, how would it be different? Young teens is when puberty starts, with the acne, not teens about to go to their high school prom. Junior high and middle school are the intense bullying years, the years of social gossip, and the years of joining groups and cliques and working on your identity. It's the years of being embarrassed around other kids and starting the rebel. We already have all of that content. I'm honestly stuck on game play that is missing, but visual representation is definitely lacking.