What size diaper is/was your 5 month old in? My son is in size 4 already and is currently going through a growth spurt.
My son is long and his legs and butt are chunky. He's about 18 pounds and I'm not sure on inches right now.
It just depends on their size/weight. My daughter is 20months about to be 21months and in size 4 but my friends baby is a month older then her and in size 6 diapers
It differs because my 8 month old is in a size 4 diaper and my 1 yr old is still in a size 2 diaper. He’s on the chunkier side and she’s tiny.