Why is Gretl 18+?

Because the math doesn't work otherwise. Gretl computes your target with the Mifflin-St Jeor equation, which was derived from adults and has no term for growth — a body that's still growing needs energy the equation doesn't know about. A target from it wouldn't be conservative or aggressive, it would just be wrong, and it would be wrong while looking exactly as confident as everyone else's.

The second reason is simpler: putting a daily calorie limit in front of a teenager isn't a product we want to build. Both app stores list Gretl as 18+, and the app won't calculate a target for a younger age.