For my stuff, it's 18-28... However, I'll look at ports of models up to 32 if they have the right look...
I routinely shoot commissioned portraits of people of all ages, but that's not applicable to my weekend projects.
For art-nude, age is less important as long as the figure is suitable, but I've found that a general rule of thumb is that 30+ models in really good shape come at premium prices (as they should), whereas younger models still gaining experience are more willing to work TFP or for lower rates.
If a good figure can be had for free and experience isn't that much of a consideration, why pay more?
Also older models, however well-preserved, still have more 'wear' on their faces which make-up can only partially disguise, so more retouch is required, which takes more time etc... Skin elasticity is also starting to deteriorate by the age of 28-30 and so harsh lighting is more likey to show up defects than with younger models.
I can be pretty cavalier with the lighting of an 18 year old, whereas I have to actually think about what I'm doing with an older model.