I have run into ageism and it did hurt the company a lot: We were developing a game in j2me for mobile phones and had to minimize the assembly's size and maximize performance (for a few animations and for the AI's turns) - we had one guy interview for the job who was almost 60, which made him 20 years older than anybody else. Instead of hiring the guy who spent a good deal of his life hand-optimizing code we hired a 20-something guy with good, but in this case, completely useless credentials. The first few months I had to reject everything he wrote, walk him through the disassembly and explain to him how and why things had to be done differently.

PS: I could refer the experienced guy to a friendly company where he was writing assembler for embedded systems until he retired.