It is definitely not a secret. Age bias is so obvious that its actually celebrated. Look at this example from Jetbrains Jobs page https://www.jetbrains.com/company/jobs/ We offer: Fascinating work in a friendly, young team
Insane. This issue is unfortunately what gets me so down about the diversity efforts happening today. We seek to empower a new group of people, more broadly representative of humanity's rich spectrum, to be unhireable at 45.
If we come to a place where there are scads of older-than-are-hireable-in-Silicon-Valley developers out there, then companies will hire them because they will be willing to work for less than their younger colleagues due to the glut in supply. Those companies will then be able to produce their products more cheaply, and the "young" companies won't be able to compete. Obviously this is an oversimplification, but the premise holds.
According to TFA, the salaries for college grads tend to slow their growth over time, eventually contracting as workers near retirement age. That fits very well with my experience - in my 20s I was focused on developing skills and getting paid more. My family was growing and I had a never-ending supply of energy and a deep well of enthusiasm and motviation to keep me going. I'm now in th early 30s, and feel like I'm established in my career. I've made one cross-country move and have proven myself in a startup environment.
I see a big difference between myself (at 31, married, with two children) and my colleagues (generally early- or mid-20s, single, no children) - I'm nowhere near as tolerant of long days and crunch times. I'm more than willing to work hard and make it happen when the situation warrants it, but I'm not willing to put in 60+ hour weeks indefinitely, and around the third release cycle that results in 12-hour days on the weekend to meet the deadline I lose patience with it.
I don't see myself as valuable because of my ability to stay late, or even to crank out code at a rapid pace. I see myself as valuable because I'm at the point in my career where I am beginning to be able to predict when a "crunch time" is going to happen a couple of weeks in advance. I can use that foresight to organize a late-night hack session or maybe a mid-day sprint before the crunch happens - so that when many of our other developers are highly stressed and trying to make last-minute changes to meet deadlines, I tend to either have my things done or I have gone back to the people who make the decisions and gotten approval for a revised release timeline or feature set.