It depends on what type of relationships and what types of stories you're talking about, but for the most part, yes.
Well, different relationships are different and people generally fall into different types of relationships based on relative age. Younger people generally form more mentor-like or avuncular relationships with older people. That's not to say romantic relationships are off the table, but they are more complicated. At the very least, the gap is likely to also bring about a gap in interests, ideals, general knowledge, and goals, which makes maintaining a relationship more difficult. At worst, it could be abusive, illegal, or just plain gross, with those involved not having enough experience to be cognizant they're being taken advantage of. Disparities in maturity can also lead to an uneven distribution of responsibilities in a relationship. Not even mentioning that people tend to change over time, and that change tends to move faster when you're younger. So when two people change at different rates and in different ways, that can also put a lot of strain on a relationship. And if the younger party hasn't reached a certain point of maturity prior to entering that relationship, there's also the chance that the older party may take on a role more parental than paramour. Relationships are complicated, people are complicated, and "is age an issue in relationships" is a very broad question that doesn't lend itself to an easy answer. But for the most part, yes, chances are that age in a relationship will be an issue in some way.
Depends on how large the age gap is for one. Adult heterosexual relationships often include a slightly younger woman some five or ten years less. That's not too difficult or weird. But say, a thirty year old with a twenty year old is more problematic because the twenty year old is still sort of an adult in training while the thirty is more mature. And of course, some age gaps are, you know, criminal. So it does matter quite a bit what the respective ages are here.
Depends on the relationship, since there is more than way to be in one. You have the lovers, friends, friends with benefits, and the lesser extent Dom/Sub. I won't mention the obvious no go, cause it is illegal. IDK beyond that where age gap is really all that significant as it pertains to writing. If it can happen in reality, it can appear in fiction no problem. There is no problem if you want to write about a 40 year old woman who happens to enjoy dating men in their late 20's and early 30's. The Dom/Sub relationship can vary from just a few years difference, to an 18-mid20's adult being taught by someone well into their 70's. Though I suppose that would be somewhere in the ballpark of friends and FWB. It would all depend on what their agreed upon dynamic is for them. But if age gap is a factor for relationships, then I think I would be the worst offender with my story. Marckus is 277 years old (275ish in the first book, but physically in his 50's ), and Corlixia is about 1003. But to be fair she has a good amount in common with a woman in her mid to late 20's as far as physicality and mentality. Graxis and Dlynvacko are about 20 some odd years difference, but that doesn't mean much considering their species lives to around 500 years. Overall I think it falls down to what people in general would consider an acceptable gap in age for a particular relationship. Lots of options out there.
I don't think a large age difference between adults is a big deal. My girlfriend is more than ten years my elder; we're a really odd couple anyway so we get evaluated on more than just the age thing, lol.
Genuinely don't understand the question. In life, either it is an issue, or it isn't. If it isn't an issue in your story, then it isn't an issue. If it is, then it is. It kinda depends on the story you're writing...
Disagree. Readers can (and frequently do) take issue with subjects, characters, or relationships that the "story" or author seemingly doesn't take issue with. Editted to add: and vice versa