This question is highly subjective (naturally) and my take is from my own selfish POV. Relatively speaking, the percentage of a two year difference in age is greater at twenty than at fifty, and so is its effect on the relationship. I believe that a narrow spread in ages early in life is important, when the partners have not experienced much of life, and the social stigmas of a wide spread create unnecessary pressures that can be hard to overcome. In middle age on the other hand, a wider spread is easier to deal with as the life experience of an older gentleman can yield positive benefits for his younger mate, in terms of financial stability, wisdom of the years, and a wealth of varied knowledge gathered.
When I was eighteen, I once dated a 20-year old woman. At that age, I could sense the age difference in her (don't ask me how), but it was there. I eventually met my first wife at 22 (she was 17), and we married when I turned 30. That age spread was just about right at age 30. As I approached my 50th year, I felt that she had overtaken and passed me on the maturity scale. Plain and simple, she turned into an old bag even though she was still 5 years younger than me. We split up!
My new partner of five years is 16 years younger than I am, and her maturity level is just about where it should be. Alas though, I fear that even she may overtake me eventually. She is now 40, but I can see the signs, and hope that my lack of tolerance for aging bitches subsides, otherwise I may be on the market in the not-too-distant offing.