VCs and in-crowd founders, not engineers, created that age discrimination culture. It's a way for the people who have made it already to put phony time pressure on the people coming up the ladder. The only way to get a highly talented person to throw 100 hours per week behind someone else's shitty idea is to convince him that he'll be yesterday's dogshit by age 40. When people are scared, they're less likely to challenge authority.

Back when the Valley was about technology (imagine that!) an age discrimination culture was untenable, just because ours is a field where it takes so long to get half-decent at it. Now that the marketing people and MBAs rule it and actual smart people are second-class citizens, age discrimination is much more prevalent.