fixed plateau age calculation
fixed plateau age calculation

NMGRL/pychron,USGSDenverPychron/pychron,USGSDenverPychron/pychron,USGSDenverPychron/pychron,NMGRL/pychron,UManPychron/pychron,NMGRL/pychron,UManPychron/pychron,UManPychron/pychron,NMGRL/pychron,UManPychron/pychron,USGSDenverPychron/pychron