I always believed Yachiyo being older than Iroha despite very clearly meant to be a couple is a reference to the way that you can interpret Homura as being much older than Madoka if you take the cumulative time of the loops in account, it's not an interpretation I agree with, Homura did not grow in any way during the loops and her apparent maturity was more accurately described
as deadened emotions to keep herself going, but the idea does exist.
As for MamiNagi, my main issue has always been less the gap and more that Nagisa would be too young to even have a concept of romance, sure, little kids do get crushes, but they have a different context to them than teenager crushes, this would not be an issue if Nagisa was 20 and Mami 26, which would be the same gap, but at their current ages, that gap comes with the implication of them having completely different levels of maturity.
...which is also not to say they can't care for each other, I do love me when people take the more wholesome route and have Nagisa be a little sister to Mami, it's just that even as an avid shipper, I acknowledge that not every single relationship needs to be romantic, and I do think that Mami and Nagisa should be one such case.