"You don't stop playing because you grow old; you grow old because you stop playing"
The book is about the 'radical plasticity' of the human form throughout a lifespan (within reason), and how much of what we take for 'normal' aging is influenced largely by sociocultural conditioning, which works like a self-fulfilling prophecy if we accept what we are told is the way things are. So play!
To my delight I recently found a giant swing at a playground in Chatswood; and immediately starting swinging on it (of course). Man, it was fun (the chains are like 4 metres long!). So.. whilst having fun (playing) on the swing I noticed a number of adults looking on with expressions that ranged from mild disapproval, to amusement with a hint of 'Dammit! I'd like to go on that swing too, but am worried about what my wife/friends/kids will think'.
If you live in Sydney the swing is still there to the best of my knowledge - why not go and check it out? You could even sneak out there at first light if you care what people think. If anything the swing was more fun than I remember it being when I was a kid, as I am heavier and stronger now and can get much higher. Cheap thrills. :)