Most forums do not allow that for, not only housekeeping reasons, but also legal.
I just want to delete my own posts, and I would have no problem with a time limit on that.
Just like it had been before.
However, since you are sort of Staff.............. I can see why you might be allotted a little extra freedom.
Also, deleting an old post, for giggles and craps, can severely screw up the database that the forum uses. SMF has been known to crash for that kind of thing.