Professional basketball, football, concertist, etc... Professions where only the very best can succeed. Maybe you can learn piano at 21 and go on to become a renowned performer, but there's usually a correlation with learning things early. Might have something to do with the 10,000 hours thing.

Pretty much all athletic, dance & music jobs. Most of my fellow students at Caltech had a history of doing projects in their field of interest as teens.

Whether or not this applies to other jobs has nothing to do with whether or not it applies to CS-related jobs.

But tech jobs in general don't expect you to have been doing it since early teens.

You can perfectly get a job in some Big Co. even if you only started programming in college.

But startups in general (and in this case, YC) all want to hire the top 1%, so isn't it natural they are biased at the people with the most experience, because they believe that they are better (which is not necessarily true)?