Sometimes, for some people, life begins much later than that.
It all depends on your circumstances and how much you've had to overcome.
Consolation to those who didn't live well before 40.
It can be true because of experience and wise after 40.
Maybe life is different or even better after 40 (I wouldn't know), but calling it "real" is silly. It's not any more real than the rest of life, unless you've invented your own definition of "life".
The word "real" is too commonly used to say your own group is the important one and you're not even going to listen to others. E.g. "the real God", "real Dutch people", "real republicans"... Whatever comes next is going to be a claim to superiority without evidence.
Life begins at 40 is the age when you have enough experience and skill to do what you want to
do with your life. I don't think that is true anymore. The stress of modern life could mean you
have to wait a few more years before finally finding contentment and start enjoying life.