I think what he said was a simplification with some truth behind it.
At the start of life as a newborn you are utterly selfish for evolutionary reasons and you don’t have any concept of putting your own needs aside for the benefit of others. As you get older you spend more of your time working towards common goals with others. When you are an adult you generally have people who depend on you (young or old) and your effort is largely directed towards their welfare. As you approach the end of your life if you are still able to contribute to society most of the things you invest your time in will not come to fruition in your life-time so you have moved full circle from selfish to selfless.
Becoming a parent is typically a watershed on this transition as the demands on you to support others increase dramatically at that point but there are plenty of people without children who selflessly dedicate themselves to others and plenty of parents who don’t.