We assemble the largest sample of oxygen-rich (O-rich) Miras and associated long-period variables (LPVs) to date and highlight their importance for age dating the components of the Galaxy. Using data from the Catalina Surveys and the All Sky Automated Survey for Supernovae, we extract a clean sample of ~2200 O-rich LPVs, stretching from the Galactic Bulge to the distant halo. We estimate ~80 per cent of our selected sample to be comprised of O-rich LPVs. Given that the period of LPVs correlates with age, this offers a new way of determining age gradients throughout the Galaxy. We use our sample to show (i) disc O-rich LPVs have periods increasing on moving outwards from ~3 to 15kpc, so the outer disc LPVs are younger than the inner disc, (ii) the transition from younger disc to halo LPVs occurs at r ~15kpc and is marked by a plummeting in period, (iii) there exists a population of young O-Miras likely kicked from the disc to heights of order of |Z|~10kpc, and (iv) great circle counts of old LPVs show evidence for distant debris agglomeration associated with the Magellanic Clouds. We spotlight the importance of Miras and LPVs in the Era of Gaia as universal chronometers of the Galactic populations.