As far as I can tell the drop off starts in the mid-30s and is very strong by 40 (which also just happens to be where one can file an age discrimination lawsuit which is nearly impossible for not being hired but much easier for being fired). And that's what I personally noticed in the D.C. area in the '90s due to fortunate genetics (until about 49 and 1/2 a few months ago when I started to get some gray hairs I was still frequently being mistaken for a lower 20-something college student): at a certain point it became a lot harder to find work until I got a clue and scrubbed my resume of all evidence of how old I was.

(One interview was very amusing where I slipped and started talking about working on PDP-11s and the guy exclaimed "How old are you?!?!!!".)