Edit old comments.

I'm unaware of any lock on editing that is time-based.

Each post by you should be editable unless perhaps the whole thread is locked (which is a possibility).

Also keep in mind that you cannot edit what was quoted by you.

Perhaps the best long-term solution is to not post things that you would regret or don't want close friends reading. This goes for everything on the internet. I think a general good rule-of-thumb is to not post anything to the internet you wouldn't want your closest associate to see (your mother, father, friends, etc).

I think a lot of people lose track of themselves in the anonymity of the internet and don't take the appropriate time to think about what they're saying and who might end up reading it.

Good luck.