system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do we know how 'old' light is?

How can we know that there is light that is 13 billion + years old? If we are still receiving it and sensing it, how can we date something that doesn't have tangible mass?
gpt
We look at where it's coming from. Say, we're trying to get a snapshot of shortly after the big bang, our theory predicts roughly what it should look like, and roughly where we should see it, so if we something resembling what we're looking for in the place we're looking for it, then we can assume the light is that old.

But if you were given an individual photon and asked to identify its age without any further information, you wouldn't be able to.