Actually, you should change when the apron is no longer safe.
As long as the apron goes high in the back AND the baby and carrier are comfortable, there is no emergency.
When the child begins to be less comfortable due to the height of the apron or the width of the seat, a change may be considered.
In general, the Toddler is optimal around 2 years of age, which is around 32 inches (81-82 cm) in height and about 25 lbs (11.34 kg).
Despite all these indications, it is strongly recommended to try out a Gustine Toddler to validate that the fit and installation are safe and that the apron has a good height.