Not a YC partner, but I help review apps... when I look at age, it's not so that I can discriminate based on it, it's so that I can see how impressive a person's accomplishments are in light of how much time they've had to achieve them.
For example, I know of someone that changed careers into the tech industry in his mid-30s. He should not be disadvantaged because he entered into the industry later than peers.
Additionally, I can think of countless examples of "slow starters" that didn't really get good at something until there was something that caught that person's interest.
At best, this is a flawed metric with many nuances to consider. So many that you could probably make the same determination by removing age from the application.*
* If you want to collect demographic data, you can just ask for it in the submission form but not show it in reviews.
I'm not a founder [but that doesn't mean I wouldn't potentially found a company someday] - I worked in hospitality management until my early 30s. If I were to apply to YC I wouldn't expect you to be interested in my previous career so I wouldn't include that. If I understand you right you're saying you'd look at what you see I've done [in the ~3 years I've been a developer] and be less impressed because it's coming from someone in their mid-30s instead of their early 20s.
Sorry to break it to you, but factoring that into your decision is the very definition of [age] discrimination.
Sorry, I didn't mean to give the impression that I only look at technical accomplishments. I like to look at whatever accomplishments that person has, regardless of what field they're in. So if you've worked a lot in hospitality management, then great, let's hear about your accomplishments there.