Pretty much the same as the OP - it was 1978, I was 16, me & a bunch of schoolmates decided to form a band as we walked home from what was the first gig I (and probably they) went to, having underaged our way into a boozer to watch a bunch of local heroes. I was happy to be bassist, as my listening included The Stranglers, Motorhead, The Who, The Jam etc, so upfront, aggressive bass playing was very much on my radar.
The decision to start a band was really just the result of being a bit the worse for wear after a couple of halves of bitter shandy, nonetheless a few weeks later I got a bass & started to learn. Several of the others ended up in bands, although none of us ever actually played together!