There is a forum restriction in place to prevent people from editing their threads/posts that are older than X days.
This does scale with how new or active you are in the forum i think.
It’s tied to your “trust level” i think, which you do accumulated by posting and interacting with other i think.
I don’t know the exact numbers, but the age of a post/thread can be to be still editable does scale up.
Also there is a restriction on how many symbols you can change.