But there are a couple of small ones -- I have had SBs from pretty young (a 21 and two 22-year-olds) and pretty old (a 38, a 39 at a couple of 36-year-olds). In my experience, there isn't a perceptible difference in cost, ability to connect, or enthusiasm if there is good chemistry. I also haven't seen an age-related difference in communication skills that can't be explained by education, the quality of jobs they have held, and correlation to other personal habits.
I have noticed two real differences that I mark down to age...first, older women do bring some real life baggage to the arrangement -- kids with schedules, a life-time of shitty men behind them, efforts to climb the work ladder, etc. None of these are necessarily bad things, but they can complicate timing, relaxation and mood at dates, and overall scheduling. This is not true for all women, but just in general I have found that older women's more complicated lives clearly make the arrangement different. And there can be good come of that -- arrangements with single parents with real jobs can be very efficient, predictable and reliable -- if you are a good SD, she doesn't have time to search for your replacement and tends to take care of the relationship.
The second real difference is that younger women are very open to being taught about sex, and older women want what they want -- they have sexual habits and need/want what THEY need and want. Again, this is not true in all cases, but in my experience, it's a good generalization.
I highly encourage you to jump into the older waters...the 30-somethings are different but very fun.