@brandon in the early to mid 1980s probably, though the concept wasn't formally established yet. I was a fan of the type-in BASIC software in publications like COMPUTE! And Antic.
I was formally introduced to the concept in around 1994 when I was at the University of Alberta by Kees den Hartigh who kept the electrical engineering labs running. He was already a huge FOSS evangelist and v1.0 of the Linux kernel had just been released.