A mutation or virus (we do not know) has suddenly modified humanity by making people live indefinitely though cycles of aging (to 70 yo) and then rejuvenating (to 3 yo).
This does not make people immortal: they are still vulnerable to typical accidents, we just rule out the death by aging and illnesses. The birth rate had homogeneously dropped worldwide so that it compensates these accidental deaths. This way the world stays at today's ~7 billion inhabitants.
People are spontaneously aware of the change (an alternative could be to have them discover this as they go, I believe this would just be an initial perturbation leading to stability equivalent to them having been made aware of the change spontaneously).
People keep their memories through the cycles (their experience is cumulative across the cycles).
How would that change our world?
This is an idea I had under the shower so feel free to make reasonable assumptions on what I could have missed