What happens at 40...?
It's both mid-point of your lifetime (not actually, but the preconception is that most people die close to 80), and it's the start of your body's health decline, so it's a good time to do some of the more ridiculous things you've wanted to do.
A lot of people go for a climb on the Harbour Bridge or get driven around a race track in a V8 Supercar with a famous driver for their 40ths.