You can't use age (resp. gender) as a factor in your decision making (bloody difficult to prove one way or the other most of the time I imagine) but that doesn't mean you can't run jobs4oldies.com or Women's Careers Day with Special Guest Sheryl McHighflyer.
That doesn't mean you should, but I'm pretty sure it's not illegal (in the UK, or generally against the spirit of similar legislation). IANAL.