Well for me, I don't really look at the ages of folks anymore. I really look to see if there is any kind of an attraction physically and then socially. If I can get along great with the other couple and they are attractive to me, then what difference does it make if they are in their 20s or in their 50s or 60s?
MrsVan does have a very loose age limit at around 40ish. Now that isn't a firm limit as she is very similar to myself, in the fact that she places a higher priority on if she is attracted to the couple and if she can get along with them.
I think age ranges are ok so that you can have a rough idea of what you are looking for, but I would say, don't limit yourself just on their ages.
And yes I can say for sure that most middle aged folks would look at a 23 year old lady and be attracted. Especially if she were much more mature than the normal 23 year olds.
-Van