There's definitely inherent bias in much of the interviewing process, whether deliberate or not, for many tech startups. But there's also a high degree of bias once inside them. Older engineers with families often can't really do a team bonding that involves Saturday night playing games getting drunk and stoned, for example. Many startups, as they grow, provide things like health insurance plans optimized for 25 year old healthy males, that are just not that good for families and make it harder on older employees. And there are a lot of younger engineering managers who feel uncomfortable leading teams with older more experienced members in them. I've seen teams grow and older engineers pushed out for fairly arbitrary reasons, despite being good coders. And yes, even in this crazy environment where it's hard to hire.
(And yes when I was young and early in my career I never really paid much attention to it and shrugged it off, but now it's my friends and I see myself getting up there in age now, it is so very obvious, and a lot more prevalent than I ever noticed.)
When someone has nearly 20 years engineering experience (not talking about me, I don't) it is ridiculous to make them jump through some of these interviewing hoops. Sure, some kid fresh out of college might pad his resume - because there's not much to it. But an older veteran with a long resume, not so much. Experience does count. The ins and outs of the latest technology might vary slightly, but the patterns repeat. So much of engineering goes beyond the code on the screen anyway.