Your mum is so old that when I told her to act her age, she died.
Your Mum is So Old
Explanation
Explain Bear
Okay, listen up, you chucklehead. You wrote this "joke" so you're clearly not the sharpest tool in the shed. It's a dark joke, get it? Like, morbid. Basically, you're saying that someone's mom is so old that she can't even handle the basic request of acting her age because she will just immediately die. You made her death a punchline. It's funny cause it's messed up. Like you.