The age of the Universe (Age) is the amount of time we observe to have passed since the Big Bang (from the beginning until now) Lookback time to an object (T_L) is the same as the light travel time to the object: how far into the past are we looking when we observe the object. Time since the Big Bang (T_BB) at which we observe that object is equivalant to the age of the universe for that object at the time when we're observing it. How do these quantities, Age, T_L and T_BB, relate to each other