If the kids are under about 6 and both of "team ability" (obviously if we're talking about two completely different ability sets, it's a different story), I think as a general rule, yes. A year makes a big difference at those ages, even just in terms of their ability to focus throughout the practice, etc.
Once they get older and are competing, it really depends. An older girl who has a lot of talent may get some things faster than a younger girl who is also talented...the routines are easier to pick up, and they might be able to use the equipment more efficiently (better approach for vaulting, easier to swing bars in dowel grips). Maturity wise she may be able to process corrections more quickly. But after a certain age, body type can come into play. It's probably harder to learn some things after you get taller, that you might have been able to learn had you started younger. But it's hard to draw these conclusions. We can generalize based on trends, but there's no counterfactual.