Ian was born in 1942, so when he and the Blockheads wrote this song in 1979, he was the grand old age of 37 years old. Given he was at the height of his musical career at this point, after a fairly late start, I would think it fairly obvious that “going on forty” was a reason to be cheerful for him.
It’s been a long time since I was 40, but I was on a number hunt over the last couple of weeks for my photo site and did happen upon this great 40lb dumbbell in the gym which I offer as an interpretive image.