If everything in the universe is radioactive then the decay equation is given by
where
is the number of atoms remaining at time=t and
is the number of atoms at time=0 and
is a constant depending on the substance.
this exponential decay can be transformed to give an equation for time as
if the
can be applied also to living organisms then all our betas should give all of us an average life span of about 100 years give or take a few years.
For human beings,
would be the initial number of brain cells (neurons) at birth.