Not to be critical, but this area of inquiry has been covered many times before, widowmaker.
But I always like to prattle on, so I will say I prefer a younger sp, and have seen sp's as young as 18-19, with ideal being 21-22. I consider an sp even in her late 20's to be on the older side. For me, it is simple. Younger women are typically thinner, more toned, and therefore more physically attractive to me. If I am going to pay for it, I want to be as attacted to the sp as possible. I admit it, I am shallow that way.
I am in my early 40's, and truth be told, I was not a big hit with the ladies when I was younger. So another reason why I like the younger ones is because it makes me feel like I am making up for my wasted youth.
Another reason why I like the younger sp's is the exact reason why some hobbyists do not. Some say younger sp's lack polish, sophistication and sensuality, and that is exactly why I like them. I enjoy the freshness, relative innocence, and "I have my life ahead of me" quality that young sp's have. I especially like it when I can get high mileage from a younger sp and/or convince her to try something she hasn't before, like greek. I am realizing now I am sounding more and more like a dirty old man. Oh well.
That all said, I am beginning to think more and more I should be open minded and see a few older sp's. Maybe I am missing out on something. But truth be told, there hasn't been one who has really caught my eye yet.