Hi Harold and Ann. I'm not part of a couple; nonetheless, since I "just entered my fifties" about 3 years ago and became interested in sexual exploration shortly thereafter. I thought I would make a few comments regarding your specific question as well as a few unsolicited comments.
I don't believe you will find as many new people in our age range as in the younger age groups. One of the advantages of our ages, though, is that most of us have attained a comfort level with who we are and where we are in our lives (both in and out of the lifestyle). It has been my experience that that comfort level is instrumental in my abiltiy to enjoy myself with others and to form the type of relationships that I seek; i.e., the more long-term relationships that are based on friendship as well as sex.
Certainly, most of us in our 50's don't have the bodies or the stamina we did 20 years earlier, we cover the grey hairs, etc. (I'm certain you know the drill!
!). But we do have things to offer that the younger groups don't. And occasionally, you may even run across a younger couple that has the level of maturity and interests you seek. I would never discount a couple merely based on age, but I have found that my interest, sexually, is geared primarily towards those close to my own age, just as it is in every other aspect of my life. While I may enjoy going to lunch with some of the "younger kids" at work, I tend to gravitate towards the "older kids."
I, too, was quite concerned about my age when I decided to go for it; however, I have found it to be like much of what we concern ourselves with. Basically, a non-concern. There are enough of us out here to meet the desires of everyone - different cultures, different races, different ages, different sexual interests...and the list goes on. Just sit back, relax, and know that there is a 40-50 year old couple looking for you. And enjoy yourselves!!