Because the amount of radioactive sample left at time t, N(t), is related to the initial amount of the radioactive sample at time t=0 by the law
[tex]N(t) = N_0 e^{-\lambda t}[/tex]
where [tex]\lambda[/tex] is the decay rate of the sample. If we are able to measure the fraction of sample left at time t, [tex]N(t) / N_0[/tex], we can estimate its age t, but only if we know the decay rate of the sample, [tex]\lambda[/tex]. Without knowing this value, we cannot estimate t from the equation.
