The age of a model is a statistic of that model. It's no different than her cup size, hair color, or height. Models get cast based upon their ability to pose and model and their stats. Sometimes age is a relevant statistic and sometimes it isn't, just like hair color may or may not be important for a particular shoot, that's all there is to it.
Sometimes having a younger client is more important if you want to portray youth. Sometimes an older model is better because, if she's been modeling for a while, she may bring more experience to the shoot than a younger model. Every shoot is different and every look is only relevant to that shoot.
I just did a shoot with this awesome model, Monika. She's 34 and looks amazing. If she were 24, it wouldn't have made any difference but at 34 she was a pleasure to work with and nailed this shoot.