A1-A4 are all acceptable, but I’d personally never say “Oh, I’m sorry” in this situation, since that sounds a bit too severe and formal of an apology for such a minor error. If I were apologizing directly to Meg for getting her age wrong, then I might say “Oh, I’m sorry”. Usually people aren’t that upset if you think they’re younger than they really are, it’s the opposite that gets you in trouble.
A5 could work if you said “I get that wrong all the time”, or “I always get that wrong”.
For a slang term in a very informal situation, I might also say “My bad” (- but only if talking to people my age or younger, as this is relatively current slang). I might also say things like “Oh, I stand corrected (then)”, or “(I) guess I was wrong/off/mistaken (then).”
If you wanted to express disbelief, you could say things like “Oh, really?”, “No way?”, “Are you kidding/serious?”, “Is she really?”, “Is that so/right?”.
“No way” is rather informal usage, as is “Are you kidding/serious?”.
There are of course profanities or oaths ranging from mild to extreme which a person could flavor their statement with also. That would depend upon the social context of the situation.
