I'm 30 years old now and find myself in a similar situation. Among the three categories you mentioned, the "significantly older than me" crowd are the most appealing. I'm actually finding myself developing feelings for someone possibly in their late 40s, early 50s. I know it can't possibly go anywhere. I quite agree with your cousin that dating someone even ten years older than me would be pushing it. Ugh, I don't know either. I'm still sussing things out and needed to vent a little.
I'm the same age as you, and I am kind of curious about your situation - up to you if you feel like sharing more, of course. For instance, how do you know it can't go anywhere with this guy - is that in terms of the age thing or are there other factors at work?
I've never exactly had an "older guy thing"; well, a lot of the celebrities I've had crushes on have tended to be in the "at least twenty years older" range, but I usually appreciated pics of them when they were younger, too.
Mostly I wouldn't have seen myself interested in anyone more than ten years older, though that I could have seen fairly easily, or at least eight. Life can take you off guard though...I did date a guy four years younger than myself, when I was 24 and he was 20, and I would NEVER have seen that happening. Didn't work out well - I think I'd be a bit wary of several years younger even now that I'm 31. But then, you do just have to look at the individual. But in real life most of the men I've been either interested in or involved with have been either right around the same age as me, or about 6-7 years older - as well as the four years younger guy, which was definitely an exception.
I did have a major crush on a man 14 years older than me in recent months, which I thought had a decent chance of being reciprocated - brought to a rather painful end when he told me he'd started seeing someone else. Turned out she was only a couple of years older than me, which didn't make me feel much better either. (There are a few threads floating around that I've started either directly or indirectly related to this guy!).
In terms of what was very appealing, some of which has been touched on already: he had a wide range of interests and was self-made in terms of employment and education; funny and charming; more gentlemanly and helpful than most guys my age I know; comes across very confident; very hard working and motivated; genuinely very responsible and a loyal friend; etc. The fact that he's handsome and doesn't look more than 38-40 didn't hurt either.
Less appealing: seemed to have some hangups in terms of disparities between our educational levels and social class backgrounds, and general insecurities in such areas (I think the confidence was partly in aid of hiding some significant insecurities); similarly, somewhat hung up on how "old" he was getting (while clearly aware that he's still very attractive, in better shape than many much younger men, etc); a somewhat bitter/disparaging view of women his own age; possibly too charming and flirtatious for his own good; "hard working" with him seems to be borderline "workaholic"; is aware and admits that he has issues with anger management (it was one of the main reasons why his fiancee broke up with him), but still has a hard time controlling anger and aggression - I was on holiday with him and a couple of friends and he flipped out a couple of times, like arm-waving and shouting, over situations which were only mildly frustrating, while the rest of us stared at him in disbelief - then he didn't seem to think he'd flipped out at all. On another occasion he had totally laid into one of my friends when she was taking an unwise course in life - like tough love to an unpleasant extreme, also involving yelling, and anything he said about that later also implied that he didn't think he'd been unpleasant. That's the kind of thing which you can't expect to change in another person, and especially not by the time they're 45. It was a weird mix of being self-aware and totally not self-aware.
Much of what I've thought about this guy was speculation as we didn't actually date, besides going out as friends and that sort of thing, but I did get to know him fairly well as a friend. He's probably an ESTJ, by the way. I think the interesting-ness and the general responsibility and loyalty were at least partly due to him being older and those would have been great. However, he's not sensitive to emotions and there's the anger thing too and it's 99% sure by that age that those things are not going to improve. I think he had picked up a good deal more scar tissue in relation to past relationships than I realized, and I also tend to think he would want a good deal of control in a relationship (partly due to having felt hurt and betrayed in the past) and would definitely want to be the power-holder. While he seemed to admire the fact that I was laid back and had my life together - which made me think he might be interested in me - it seems from what I heard about the new gf and about his past relationships that he prefers women with plenty of "issues" (whether emotional or health-related) and though it might be unfair of me, it did make me wonder if he wanted someone to "rescue" or someone who would be very dependent on him and hence couldn't talk back too much. I do think that could have something to do with going for someone both younger and with issues, which seem to be trends in his relationships.
Anyway, sorry, that's only partly related to the age thing, and partly to this specific individual, but I hope it's of interest. The whole thing has definitely been on my mind lately!
By the way, especially based on the title of this thread - do you feel that an "older men thing" is particularly INFJ? Or are you going more for how INFJs get into these situations and how it might affect them particularly? I wouldn't have thought that having a thing for older guys was particularly related to any specific type, but I'd be interested in what others have to say!