I'd consider it as 5+ years, but others may think differently. That's another issue with this kind if stuff.
However, if both in the relationship are adults then there isn't as much of an issue (or even an issue at all) as they both are fully aware of what they are getting into. It's when one is under 18 that it gets into the region of needing to be checked due to the possibility of illegal activity.
But hey, that;s just my opinion.