DS#2 started wearing size 4 at ~1 1/2 years. He's 2 1/2 now & still wears the size 4 to bed. He only moved out of a size 3 because he needed something more absorbent. He has no butt so he can stay in smaller sizes for quite some time. He's potty training & will never wear a size 5.
DS#3 is turning one in two weeks and he can wear a size 4 no problem, but he still can wear a size 3 no problem. He has a huge butt, so I think he'll
need a size 4 in a month or two.
DS#1 (normal size butt) moved into a size 4 at about 9 months (wore a size 18 months, almost 24 month--very tall & lanky). He potty trained at 2, and was in a size 5 (barely) at that time.
My kids fly through sizes newborn - size 2, but hang out for quite some time in sizes 3-5.
I think more of it has to do with butt size & absorbency need rather than age & weight. My kids are always 50% lower in percentile for weight than in height. DS#1 100% height, 50% weight; DS#2 is 30% height, <5% weight; DS#3 is 70% height, 20% weight. If they weigh less, they probably pee less. And well, I mentioned their butt size above

. Hey, if it can't cover, it isn't doing its job as a diaper and if their butt is small, it's going to cover longer.