Anywhere from 18-35 is usually the age range that I like.
I agree with GSB... the usual preference for myself has been the typical age group but there are exceptions who continued to work in the industry whom I still find/found unbelievably gorgeous, if in some cases less so than in their youthful newness to the industry.
Having said that I think once a favorite is established, at least for myself, they are likely to remain there unless they do something to pressure my decision to keep them on the brain.
I always loved Jill Kelly and believe she was quite a beauty the last I've seen of her... I guess I do however get my head turned by the hot newbs to the industry...
Taylor St Claire is another example of a beautiful woman who may have gotten better with age, although I haven't kept up with her current situation... I heard Raylene's coming back and she still looks quite fine in my opinion.