  Accurate determinations of stellar mass functions and ages of stellar
populations are crucial to much of astrophysics. We analyse the evolution of
stellar mass functions of coeval main sequence stars including all relevant
aspects of single- and binary-star evolution. We show that the slope of the
upper part of the mass function in a stellar cluster can be quite different to
the slope of the initial mass function. Wind mass loss from massive stars leads
to an accumulation of stars which is visible as a peak at the high mass end of
mass functions, thereby flattening the mass function slope. Mass accretion and
mergers in close binary systems create a tail of rejuvenated binary products.
These blue straggler stars extend the single star mass function by up to a
factor of two in mass and can appear up to ten times younger than their parent
stellar cluster. Cluster ages derived from their most massive stars that are
close to the turn-off may thus be significantly biased. To overcome such
difficulties, we propose the use of the binary tail of stellar mass functions
as an unambiguous clock to derive the cluster age because the location of the
onset of the binary tail identifies the cluster turn-off mass. It is indicated
by a pronounced jump in the mass function of old stellar populations and by the
wind mass loss peak in young stellar populations. We further characterise the
binary induced blue straggler population in star clusters in terms of their
frequency, binary fraction and apparent age.
