Like others, I also have friend of all ages, and don't see older or younger ones as 'the other side' or expect them to have 'alternative' views - sometimes they have a different perspective because of life experiences, but on the whole people are individuals with minds of their own*.
I have no issue with 'younger non-grans' except when they use their comparative youth as a stick to beat us with (which not all do, of course).
Ageism is rife on Mumsnet and is getting worse on here, and is not picked up on often enough IMO. Comments about people thinking as they do because they are old, or that a point of view is held by younger people so must be more valuable just because of that are lazy and reductive, whether they are said by older or younger people.
I'm sure that there will be younger people on here who don't shout about it, too. It's only when it is used as a badge of honour that anyone knows, unless there is a thread asking for personal details and someone points out that they are only 32.
*Having said that, I can understand younger people wanting to see how MILs think, for example. I don't see my DIL as being 'the other side', but I do read threads on MN about MILs and notice the sorts of thing that annoys DILs so that I can avoid doing them if possible. IL relationships seem to be much more fraught than they used to be, and it is instructive to learn how to avoid potential conflict. Being a MIL is a learning curve, and I think that is sometimes forgotten.