Not postgresql but I saw a start date and an age in days columns in MS SQL server. The age gets updated daily. It didn't sound right to me. I'm pretty sure I'd fail my database class in college if I did that. What is different in real life and why didn't they teach me this in college?
I obviously can't answer for this particular case, but my first thought on why I would consider doing such a thing is if I had an app in which number of days old was something that had to be queried, displayed, and/or used in other functions/queries a massive number of times per day in the course of normal application usage. If the application had low usage, or number of days old was infrequently queried/displayed/used, I wouldn't consider it. The moment I found that significant time & resources were spent calculating the value in normal/regular usage, I'd start looking at ways to reduce that time & resource usage. How to go about it varies, but the win of that value being immediately available without computation could mean a lot to an app/business and its users.
