In reply to Pero:
I'm not sure I fully understand your question, however: In getting from the current value of the Hubble constant (as you say, around 72 km/s/Mpc) to the age of the universe, one does take into account how the expansion rate has changed over time. One can do that because, given the constituents of the universe, one can calculate how the expansion rate will have changed (e.g. gravity slowing things down).
> Finally, if the universal expansion is constant (75km/s/MP), then the calculation will give a
> constant 13 billion years regardless of how old the universe is.
I'm not sure I understand the question. Given an expansion rate, one then runs things backwards in time to predict the time when all lengths were zero, so everything was in the same place. This was the Big Bang and the beginning of our universe. Thus the current expansion rate and the time since the Big Bang are intimately related.