Modification date for group - how is it arrived at

Should it not be the most recent modification group for items in the group?

More description below:

I have a folder / group of sub-groups, files within sub-groups, like so

group a
-- sub-group a1
  -- a1f1
-- sub-group a2
  -- a2f1

If I edit a1f1, the modified date of a1 does not appear to change.
Would like to be able to sort a1, a2 sub-groups, based on whichever sub-group has the most recent modified file.

Could not find the answer in the discussions here, nor in the help.

Thank you.

The modification date of groups is independent, e.g. adding/removing items to/from a group updates it.

Ok got it. Thanks for the reply.