Anyone can be old enough to remember the song if you don't limit your music consumption

Edit: "one" to old

Also, the age of aquarius starts somewhere around 2600. Or if you're not really into actual astronomy, but instead prefer nonsense, somewhere between 1962 and 2024, depending on how you define the word "in".

It's originally from the 1967 musical Hair, 5th element merged two songs from the musical and released it in 1968, that's the version you linked.

https://en.m.wikipedia.org/wiki/Aquarius/Let_the_Sunshine_In

The 1979 film version:

Warning, hippies as they were imagined in the late 70s: https://www.youtube.com/watch?v=BTZArvbmG_o