I've been playing guitar since the early 80's so that's not as easy a question to ask. I mean, I was a high school kid wanting to learn songs I liked. I can't say I recall back then my ear for how I listened to music much changed. I was just trying to learn some riffs and all that.

With that said....

While my ear for various parts did evolve, the moment I started really hearing parts was a day I was listening to Kansas 'Carry on Wayward Son'. One of the best produced songs of the 70's, I'd posit. It was that clarity that I could 'see' each part. Not just as a collective but as parts too.

Late in the 80's, I'd help run sound for some friends bands and with a couple of tips from better sound guys and a decent ear, with enough messing about, you start to realize the textures and layers. Then there's that time I was helping my friends band doing a bridge gig over a river and we got a complaint from 30 blocks away. Good times......

To the subject at hand, being able to her what the bassline does or the differences in guitar parts and where the drum falls in the groove. Hearning those parts really helps understand the 'orchestration' of a song.