When to lower crib mattress?

Shelly • Mama to 👦🏼 Jesse 6/4/19 & 👶🏻 Jameson 3/11/22

After my son just woke up from his nap he looked so big in his crib! He gets up on his hands now (not his knees) and will probably be able to sit unassisted within the next month (He’ll be 5 months on the 4th.) When are we suppose to lower their cribs halfway? Is it not a hazard until they start pulling themselves up or is it when they can sit?