I was today years old...

 ... when I learned what "today years old" means.

The phrase came up in this morning's Los Angeles Times crossword puzzle, clued as "Age of Enlightenment?"  I had to work around the clue to solve the puzzle, but even then could not suss out the explanation.  I found the answer at Know Your Meme:

"I Was Today Years Old" is an online expression typically used in the title of a post or discussion thread when introducing an interesting fact or trivia that had been previously unknown to the poster, in a similar vein to the phrase "did you know?" and Today I Learned.

On September 10th, 2015, Twitter user @stacilyncharles tweeted the first known usage of the expression saying, "I was today years old when I learned I had been wearing the wrong size bra my entire life". After June 17th, 2018, the phrase went viral after Twitter user @gabrielalvper tweeted an image of a Staples saying, "I was today years old when I found out the 'L' in 'Staples' in really a half open staple". The tweet garnered over 70 likes in two years.

But even with that information in hand, I still couldn't parse how the phrase gained that meaning, until I found the following "explain like I'm five" answer in a Quora thread:

Let’s say I tell you something you knew already. For example, “Did you know that giraffes have blue tongues?” You might respond “I was 12 when I learned that.”

Now, let’s say I tell you something that you don’t know, that genuinely surprises you. “Did you know that human being share 98% of their DNA with chimpanzees?” A humorous way of indicating that you’ve just learned that is to say, “I was today years old when I learned that.”

Not sure why this escaped me for so many years - presumably because its primary domain is social media, which I am not "on."  You learn something every day.