I'm bi, but with a cisgendered woman. She's 2 1/2 years younger than I am. Typically, I'm with AmySmiles--- I like to stay within 5 years either way. My ex-wife was 5 years younger than me, which caused some problems, particularly early in our relationship (she was 19, I was 24 when we were married), but I really think it depends on the person. I'm 29 now, and I don't plan on being single ever again... but if I was, I'd likely go for someone younger, since I've never really had a good relationship with anyone older than myself.
I do think there's often a difference between gay and straight individuals on this topic--- I have a friend who's lesbian, cisfemale, and always dates people at least 15 years older than herself. I've known many other gay and lesbian folks who were fine with a large age gap. I think what it comes down to is that when you're straight or bisexual (or pan--- semantics), you have a far larger pool of potential mates than do people who are restricted to one gender for partners, and you have the luxury of being picky. Some people just naturally prefer older or younger mates, of course, but I think the aforementioned is why we see it as more prevalent in the GLBT community.
As per friendships, I don't care how old or young the person is--- age matters far less when I'm not planning on spending the rest of my life with the person.
