You will find out by:
1. Observing baby on the ground (outside the baby carrier): if he lifts his feet off the ground while lying on his back, around 3-6 months, you can remove the booster.
2. Observing baby in the baby carrier: if when you carry in front, the baby's head protrudes from your chin and obstructs your vision even if the baby carrier is lowered as far as possible on your hips.
3. Observing the size of your baby and the baby carrier separately: You can also measure the length of the 2 thighs + baby's crotch which must be longer than the width of the central part. (The width of the central part of the baby carrier will therefore not impact the width between the two knees).
Be careful, your booster can be used again quickly, when carrying the back (to raise the height of the carry, which allows to satisfy the curiosity of the baby allowing him to see over your shoulder.
For the head support, you can keep it in place at any time, during the entire period of use, because it allows you to extend the height for a growing baby.