It seems that you can’t edit older posts after a couple weeks. Why is that?
Likely because this is a default setting for discourse and the admins didn’t change it. (default should be 2 months I think).
The admins could change it Free to edit post at any time - feature - Discourse Meta
Generally I think it is a bad idea to edit posts after a long time in a support forum. It is just confusing and valuable information might get lost.
If this is about the Tips and Tricks category (is that why you ask?): Maybe there there is a way to set the edit time limit per category.
Also there is a wiki function that might be useful in this case.
Actually it looks like there is and is only 1 click away.
@sitter can you enable
Allow unlimited owner edits on first post in the Tips and Tricks category settings? (for reference: Free to edit post at any time - #43 by JammyDodger - feature - Discourse Meta)
That would help people update their outdated information.
For example: Setting up Arch for and Building/Running Plasma 6 - #4 by Echoa
Unlimited edits are now enabled.