The absolute worst experience was a technical phone interview that was going very well, I thought I connected with the interviewer(2). So I was shocked after being complimented on the breadth and depth of knowledge I had, near the end of a 45 minute conversation, there was a pause and he asked "so, when did you graduate from high school?" Needless to say, the tone changed somewhat and I didn't get the in-person interview for the position that "wasn't really a good fit for me," and he, "wasn't really sure [we're] going to actually fill this position," forgetting that he earlier said he wanted someone as soon as possible.
(1) See Logan's Run -- it's a documentary about the Bay area companies' solution for aging software engineers. /s (2) And I knew him, but only in the sense I knew him from several mailing lists stretching back to the days of USENET as a knowledgeable, respectable guy with a good reputation.
TL;DR ageism is a thing everywhere.