I have an old post that I can no longer edit. Is that because it is old, because of the number of likes on it, or some other criteria?
Probably because of this time limit setting:
Yes, is because it’s more than 2 months old, as per @Fabe56 link.
Now that I have been knowing that, I usually turn my opening posts to wiki, when I think I will need to update it from time to time.