This all depends on how old you are. For someone in their 20's, the "cut off" age will likely be much younger than it is for someone who is in their 30's, 40's, 50's and beyond. My man and I are 14 years apart, and it does not present a problem for us. I am in my 30's and he is in his 40's.
I once heard a formula, I think it was half your age plus 7, or minus 7, I can't remember. While entertaining, I don't think such a "rule" should be a determining factor. Just as someone said upthread, it's only too much of an age difference if it's illegal or one of the partners is uncomfortable with it.