I saw on someone's profile that they would not accept any guests over the age of 30. There was no explanation given. Nor was the age of the member given. I found this age discrimination to be somewhat offensive. If someone with such a restriction on their profile asked me to host them, I would refuse and tell them why.
I realize that hosts can decline requests for any reason, but there are some reasons I don't consider valid, and age is one of them.