Vincent was 27 if I recall well when Hojo experimented on him waaaaay back then. I'm not sure how old Sephiroth is supposed to be, most likely in his 20's, so by the time Cloud find him in the basement of ShinRa mansion, Vincent is in his 40's, probably even early 50's, but doesn't look a day past 27 due to his condition. He'd be old enough to be the father of half the cast, which makes the Yuffie/Vincent shipping particularly creepy. The 20 years old Playmate marrying the 80 years old billionaire type of creepy.
Now, Rikku and Auron... Not as creepy, despite the fact she's still a teenager and he's dead, yet... still creepy. I'm not a fan of pedophilia and necrophilia. And I'm even less of a furry fan, so I'd rather not imagine a Kimahri/Rikku pairing.
And don't get me started on the incestuous pairings. Squall/Laguna, anyone? *shudders* I've even seen Tidus/Jecth and Seymour/Anima. My eyes were bleeding without even reading the fics.