I didn’t learn the term ‘closure’ until maybe 1978. I’m having trouble remembering when the first time was I used a language with them. Simula maybe? It predates Algol 68 I assume (update Algol 68 had only limited closures; see comments).
The language implementor discovers them the moment he: 1) allows a function to be defined nested inside another function; 2) allows the inner function to access the locals of the outer function, and 3) let’s the programmer get a handle to the function, i.e. treat it like data. You can actually get most of the fun of closures without a garbage collector if your clever.
They had been an issue in Lisp for years, but I didn’t have any Lisp experiance until the mid 70s.
It’s interesting how many of these ideas are as old dirt.