That is the sage advice of The BrainPlace. Unfortunately, I can find no explanation for what this rule is. Apparently, it has something to do with anxiety, basal ganglia, and possibly breathing.
Whatever this 18/40/60 rule is, I’m willing to bet that I’m breaking it right now, and that of course makes me anxious, so now I know I’m breaking the rule, man! People, we could all be breaking the rule! What the hell is wrong with us?
Ahem. Thank you in advance for your help.
Having done a comprehensive search, I can only find it referenced to a book by Dr Amen:
http://www.arkanar.minsk.by/medicine/22/Change_Brain_Change_Life_Breakthrough_Program.htm
WAG it is a technique aimed at selling books by catchy phrases. Further WAG- it has never been peer researched.
IWAMHP- I was a mental health professional- and I have never heard of it. As the internet is not awash with references, his makes me think that the only way that you’re going to find out what it is will be to buy the book.
When you’ve done that, I suspect that you will have fulfilled the intent of the 18/40/60 rule- spent $18 on the book, taken forty minutes to read it and then regret wasting the money until you are 60. 
From this site: (from a list of recommendations for optimizing brain function) “Do you remember the 18/40/60 Rule — At eighteen, you worry about what everybody is thinking about you. At forty, you don’t give a damn about what anybody thinks about you. And at sixty, you realize that nobody’s been thinking about you at all! People spend their life worrying and thinking about themselves, not you!!”