I would have been six.
It would be another eight years before I heard my first Genesis song - or at least heard a song on the radio that piqued my interest enough for me to take note of the band.
Interesting point. I think I first heard something by them that set my antennae twitching in around 1976. Either on the radio, or something I overheard my brother playing. If not recalling exactly when, I do remember the two things that captured my attention - the final section of The Musical Box, and Carpet Crawlers.