India, Sept. 4 -- Look out! Incoming friendship advice. Lots of it. Because friendship, like so much else, seems effortless when you're young, but becomes a complicated beast as you get older. What happens to 3am friends when one buddy has a baby, and the other needs a pep-talk for the next day's hustle? Do old friends hold you back or push you forward? Is it worth holding on to that one guy from school who tells the same campus joke 15 years later?

These are unprecedented times - Sex And The City started off with four friends. The sequel, And Just Like That, ended with three. Even TV friends struggle to hold on to each other. Don't let that be your story. We asked 30 people - from artists to entrepreneurs, from parents to philosophers -...