What does "I was today years old" mean?

“I was today years old” is a humorous phrase people use when they learn something new that they probably should have known a long time ago. It’s often used in social media posts or memes to share a funny or surprising fact that others might also not have known. 

For example, if someone just figured out that the little hole in the pen cap is there to prevent choking, they might say, “I was today years old when I realized that the ‘alphabet song’ and ‘Twinkle, Twinkle, Little Star’ have the same melody.” It’s a lighthearted way of admitting that you just learned something that seems like common knowledge.

This phrase has become a popular way to share quirky or unexpected facts online, often leading others to reply with their own “today years old” moments. It’s used as a playful way to admit that there’s always something new to learn, no matter how old you are.