Determine when an NPC was created?

Good news finding old Roach!

There is no way to find out when it was created. The database does not store that kind of information.

However, horses with a saddle do not move. I suspect he may have been hiding (glitched underground/above ground? Or if on a MP server "borrowed")

Oh, actually the spawndate of npcs is stored in the world database Unfortunately you cannot access this data ingame... to get the spawn date, you have to open the world database (".../Worlds/<YourWorldName>/<YourWorldName>.db") with an SQLite editor (e.g. Navicat). Then open the "Npc" table and look for the npc in question. There is a column called "Spawndate" which contains a timestamp (in milliseconds), just copy that timestamp, remove the last 3 digits (to convert it from millis to seconds) and paste it here to get the actual date: https://www.unixtimestamp.com/

I'm sure I had looked for this before! X/