Honestly I just don't see how we could double the stuff that needs stored without having a kid, no matter how many years we live.
We are not pack rats. We throw stuff out or give it away. I am sure you are right though. More time equals more stuff, but I just don't see it being an issue unless something drastic happens, such as a little terror (kid).
So far, we are happy.
PS We have 2 small cars, so our garage has lots of extra space when it is needed.