  I develop a model for the abundance and spatial distribution of dark matter
subclumps. The model shows that subclumps of massive parent halos formed at
earlier times than subclumps of the same mass in lower mass parents;
equivalently, halos in dense regions at a given time formed earlier than halos
of the same mass in less dense regions. This may provide the basis for
interpreting recent observations which indicate that the stellar populations of
the most massive elliptical galaxies are also the oldest.
