Stellar age ($t_{\mathrm{age}}$)

The stellar age is the elapsed time since a star formed. A constant number formation rate on a finite interval gives a uniform distribution of present ages if every formed object remains counted, including stellar remnants. An age distribution inferred only from surviving main sequence stars generally differs because their lifetimes depend on mass.

A cosmological age must exceed the ages of its oldest stellar populations plus their formation interval. globular cluster ages use the main-sequence turnoff with modeled distances, composition, nuclear rates and stellar transport. Their errors are correlated with these inputs, so a lower age bound must propagate the stellar and distance uncertainties rather than treating an isochrone as an exact clock.^[ref]

Estimating astronomical ages from radioactive abundance ratios. A ratio evolves as $Q=Q_0{e}^{-({\lambda }_1-{\lambda }_2)t}$ for decay constants ${\lambda }_1,{\lambda }_2$. The initial production ratio and subsequent enrichment or mixing must be constrained; an observed ratio alone does not specify a unique age.