What does the phrase "this will date me" mean?
I searched for it here on "English Language Learners" and on Google but I could not find the meaning.
The phrase "this will date me" is often used when referring to something that you would/could only know about if you are old. "This" is referring to whatever example or topic you are about to say. It's almost always used before referencing some kind of outdated technology, method, or historical event.
See definitions 16 and 17 here.
"Date" (verb): to ascertain or fix the period or point in time of; assign a period or point in time to
"Date" (verb): to show the age of; show to be old-fashioned
For example, if you are going to talk about how to play music, you might say, "This will date me, but we used to have to place a needle on the physical record. Now you can just tap your phone and music will play."
It's like a kind of joke about your own age. It's equivalent to saying "this will show how old I am". It plays off of the idea that people, in general, like to appear young or youthful. The phrase is sort of "admitting" that you are old before outright giving it away by the example you are going to use.
When talking about the "undo" feature on a computer, "This will date me, but when I was using a typewriter I had to be very careful about making mistakes."
It's just like when you are inspecting an antique to work out its authenticity. Imagine you are presented with a silver spoon of unknown age.
Well, in England, silver spoons have a makers mark called a 'hallmark'. The word hallmark is used to describe the defining characteristic of something. Cristiano Ronaldo's hallmark is his super free kicks, speed and skill.
A hallmark on a silver spoon will have a number of symbols. One of the symbols identifies the marker. One identifies the date.
When we say "this phrase will date me", it means, that what is being said can identify the persons provenance. Quite like when you want to work out whether the painting you are buying is an original, you seek to establish it's provenance, and you would do this by 'dating' the art. Work out how old it is...