This is indeed impossible, since you'd have to know which input in a ring is being spent, in order to know its age, as well as know which outputs are change and which are not, to know the amount (assuming change would not be counted in that calculation, I'm not super familiar with that metric).
Once RingCT is enabled, you wouldn't see amounts at all, so not even a range could be calculated.
Your best bet would be to trust the main exchanges to properly report their volume (but then you only get volume, not age of outputs).
Edit: in fact, I'm thinking another metric might actually be a not-so-bad replacement: Monero has per kB fees which are not trivial compared to bitcoin (0.01 monero per kB). Once RingCT is merged, most transactions will be 1 or 2 inputs and two outputs (payment + change). Assuming nobody will move money just for the fun of it, due to the fees, maybe counting cumulative transaction sizes would give a good idea of the actual amount of use. That'd be closer to "users" rather than "capital", since the amounts will be hidden, but that might not be a bad thing altogether, just different.