What we all seem to agree on is, people of different ages act differently. To the extent generalities can be draw, and to the degree these are used to spot-evaluate someone you've just met (job interview, radio survey etc), this is ageism.
 It happens because it works, at least 80% of the time.   
 Its unfair because important decisions are made about people (me) with too little information. Yes I'm old, but I actually have kept learning.