I thought this a fitting QofD today as I welcome, albeit grudgingly, my entry into the Decade 40's Club! Yikes! Anyways, my age difference rule of thumb is typically no more than 5 years up or 5 years down; however, I've been known to modify it from time to time depending on what type of relationship a client or friend is looking for.
For some, they're happy having casual flings or carefree relationships that don't require a lot of emotional investment. In this case, for example, I think it perfectly fine for an older gentleman or woman (a.k.a "cougar") to date someone significantly younger than them so long as each party is honest about what they are looking for out of the union. Others are looking for a long-term, steady, committed relationship where they're either widowed, divorced or never married but looking for "the one". Typically it's easier for these folks to stay closer within their age range so that commonalities are present and there is plenty to talk about with one another; however, it's been my experience to see many older people who are still fit and active, date someone 10 years younger than themselves and share plenty of interests.
I personally believe, that these days with people looking better than ever and taking great pains to care for themselves physically , we're going to see a lot wider age range gaps within the dating community. So, how far outside of your age bracket are you willing to date?