I have to say this would never have occurred to me. I would never have thought that there is a phenomenon of age discrimination in kollels. I thought it was only in the secular world that there is a culture of youth. It seems that the Charedi world has been affected as well.
Last week's Mishpacha magazine (in Hebrew) has a long article detailing the problem.
It seems that Avreichim over the age of 40 have a very hard time finding a kollel to learn in. Some of the reasons given are:
1. It is assumed that a young person in his 20's has more zeal for learning then someone who has been learning already for 20+ years.
2. Younger people are less busy and have more time to devote to learning. Someone who is 25 has a few little kids and not s many financial worries. Someone who is 45 probably has 8-10 kids that are older and therefore he is busy trying to marry them off and deal with their problems. He mostly probably is in finanial trouble as a result. As the old saying goes "little kids little problems big kids big problems".
3. The Roshei Kollel feel that they will have more influence on a younger person. They believe that they can have a major influence on the דרך הלימוד of someone who is in their early 20's. However someone in their 40's is already set in their ways.
4. The Roshei Kollel feel awkward about having someone in the kollel their age or older. They also feel that the person will probably not follow the sedarim in the kollel and will learn what they want.
5. Someone who is still just sitting and learing in kollel in their 40's is not going to become a Rosh Yeshiva, Rosh Kollel etc. The fact that they haven't moved up shows that they are not so successful.
These are all stereotypes but there is some truth in all of them. The article points out that the reasons listed above apply mostly to small kollels. Therefore it suggests that older Avreichim should start learning in a big kollel where these issues are not nearly as problematic.