The other moderators can check me on this but I believe you only get to modify your own post for 24 hours. There are a number of reasons for this but largely it's to limit mischief. Sort of a hard lesson from the past so now it's policy for members.
It's been that way for as long as I've been on the site, I think you've only just noticed because you have a change to make.
Just go ahead and add another post.
Without adult supervision...
Quando omni flunkus, moritati.
I'm a man, but I can change, if I have to, I guess.