That's a very good point.
But I think age descrimination is deliberately NOT included in the policy.. Because age discrimination happens all the time, specifically hosts who don't want to rent to young people, worrying about their place getting used for a big party.
So: age discrimination is clearly acceptable. It's strange how this isn't mentioned though, in the text it gives examples of 'guests who smoke' as an acceptable reason to decline someone. (As if guests are going to tell you that in their inquiry, or not Instant-Book..)
Smoking is such a safe example to give. They could have just stated: "Age discrimination is A-OK with us."