Whatever the average age of the HN crowd is would probably inform a little. I'm 34 and have been a dev for about 8 years. The only code from a past job that I know for a fact is still in use today is something that I wrote in early 2017 (a let's encrypt integration for a web host). So about three years. I'm very curious to know how it has been updated since but I'm no longer at that job.