Why does it matter how old is the author?

We should judge software by the quality, not by authors age.

It's an age-old convention, a way of supporting and encouraging kids, and harmless.

HN isn't a judge of software; it's a place to learn and be curious. So people are often interested in projects that do a novel thing in a normal way, or a normal thing in a novel way. Eg, stories fascinate us because something was built by a very lean team, or a group with no money, or somebody who is an industry outsider, or a parapalegic, etc. Overcoming these limitations is a sort of 'hacking'.

There is a new trend in Silicon Valley of bragging about how young founders are, etc. along with the rather bizarre trend of bragging about dropping out of high school to "found a startup".

Totally agree. Even if I'm a teen myself I never post my age unless someone asks explicitly. Saying your age is just trying to find excuses to justify a sub-par software imo (Not saying this project is sub-par)