We may use the current expansion rate to work backwards in time to learn when the expansion began. That would be when the universe's size was zero. An equivalent way to ask roughly the same question is to discover the distance to an object whose recessional velocity is the fastest it may be, that is the speed of light. Using H = 50km/sec/Mpc, and c= 300, 000 km/sec, the most distant anything may be is 6, 000 Mpc, or about 20, 000, 000, 000 light years. It would have taken 20, 000, 000, 000 years to have gotten there. So we may call that number the age of the universe.