From an interesting contrarian rant by Astro Teller (head of Google X) and Mrs Teller. Both authors are, I believe, the grandchildren of Nobel laureates. Not sure what difference that makes.

The origins of the parenthood religion are obscure, but one of its first manifestations may have been the “baby on board” placards that became popular in the mid-1980s. Nobody would have placed such a sign on a car if it were not already understood by society that the life of a human achieves its peak value at birth and declines thereafter. A toddler is almost as precious as a baby, but a teenager less so, and by the time that baby turns fifty, it seems that nobody cares much anymore if someone crashes into her car. You don’t see a lot of vehicles with placards that read, “Middle-aged accountant on board.”