my son is 6 months and we just ordered him a convertible carseat because he's already too big for his infant seat. he's a solid boy, 22 lbs and 27 in.

I kept my infant one until my son was 1 bc it was up to 25 lbs then bought him a toddler type car seat and flipped him around fwd facing...this was 8 yrs ago tho so the laws may have changed.

I changed seats whenever my daughter was about a year old. I made sure to get one that was made for smaller kiddos since my daughter is on the smaller side weight wise.

Whenever they outgrow their infant seat, it depends on the weight limit on the type of seat