Would a big age gap between a couple not be too bad, if the younger person parents are a lot older than them plus they have siblings around the same age as the partner.
Or is it always a bit odd if there's a 15+ age gap?
it's only a problem if they make it a problem, some people will frown on a big age gap because some people frown at everything. If either of the couple get bothered by that fact then it'll become an issue, if they don't, if they remember it's only really about them, then it's all hunky-dorey!