for me, age has little to do with compatibility, within reason. in my experience, emotional and spiritual maturity seem to have little correlation with chronological age.
typically, i'd say 10 years in either direction, but i could see myself making an exception (to a few years, maybe) if there was real, deep, and compelling basis for compatibility. but i think that's partly because the older i get, the more i realize how unimportant age can be when you are on the same page with the important things, and more practical considerations (such as children and practical limitations).
also, the last couple years have really challenged my notions of age difference. a few years ago, my answer would probably be different. also, when you're younger, i think age difference creates a much bigger issue.