4 or 5.

You're just perpetually happy at that age, excitedly learning new things every day and playing lego with your friends.

Once you're grown up you're always chasing something to make you happy. And once you have it that feeling of happiness quickly fades. So you would forever chase a fleeting bit of happiness, you'd be stuck in a loop.

I strongly believe that most people would kill themselves after a few hundred years in that scenario.