I think that under 5 years difference is okay, when she is older. Anything more than that is hard for 95% of guys out there. It all seems great until she hits the big M and beyond. No offense meant to the ladies, but when they start losing their looks and their bodies. You either need to still be absolutely 100% in love with them, or well out of your prime yourself, where your testosterone is not pushing you to make babies with everything young and hot.
That is why guys being slightly older works better, because if a guy is 50 when his wife hits 45-50, they are both out of the game. If a guy is 35 when she hits 45, the guy potentially still has options to get a 25 year old.