I’d like to put in my two cents, and it may not answer your question about the age of the harp, but maybe the age of the player.
I was saying to my harp teacher yesterday that if anyone asked her if they should buy a new pedal harp and they are in my age bracket (over 50) to consider a used harp.
I bought two new harps over the last 6 years (I love them both) but
Condition does indeed count for a lot! I have harps that are 200+ years old that play just fine, albeit after restoration/conservation. Something that’s worth bearing in mind is that if you’re looking at a harp from before 1940, you should do some research on how they were meant to be strung. Putting modern strings on an old harp can lead to some big problems!