def _age_map_select(seq_to_age_tup, age):
  """Return a map from keys to the second item in age_tup, optionally limited
  to those <= the provided age."""
  if age is None:
    return {k: v[1] for k, v in seq_to_age_tup.iteritems()}
  else:
    age = float(age)
    return {k: v[1] for k, v in seq_to_age_tup.iteritems()
            if v[0] <= age}