If the actual universe is larger than the observable universe, how can we know how old the universe is?

We can deduce how old our bubble of observation is, and we then assume the other at least 10^120 bubbles (according to Inflationary Theory) are identical. We hypothesise the universe is homogeneous on the largest scale, and that's on a scale of 10^40 observable universes for every atom in our observable universe. So the sample size is rather small.