The girls in my kid's class mastered them before the boys, and I'm not sure if this is because most were taller than the boys or if it was something else physiological (or perhaps they were more interested and spent more time on them). My DS's BFF was a girl and quite tall, and she could do them way before he could (like 6 months earlier). He worked at it and worked at it and finally got to be almost good at it around age 5 but had a bit longer before he totally mastered it. He was on the short side, though, and I think that also impacted it.

OP, how old is your child?

4.5 she could do it and was pretty adept by 5.

Kid 1-- at 5
Kid 2-- can only make it 2-3 bars so far (age 3 currently)

Hand size matters to grip whatever sized bars.
Also the concept or usage of swinging/ momentum helps too.

DS 1 - 5, beginning of K. dS2 is in pk this year but seems to have no interest.

Both kids around age 6 (boy and girl)

4

DD is almost 4 (June). She is able to do 1-2 at a time.

Mine is 5 (turns 6 this summer) and has been pretty good at it for a few months. She doesn't always have the reach to do some of the more advanced ones, but as long as she can reach the next bar she's good.

A lot depends on height (or wingspan, anyway) since you need to be able to reach both bars at the same time.

At 4. She's 4.5 now and has been doing them since last fall.

1st grade which was not on the early side

At 5

I mean the ones that look like a horizontal ladder-- you have to swing from one to the other to get across? My DD has recently mastered this skill and my DH is sure she's headed to the Olympics. I'm pretty sure he is crazy.