Are ids strictly sequential i.e. does later added content always have higher id?
User IDs and Node IDs are automatically incrementing (mysql, postgresql, sqlite) fields. Once an id has been used, it should not be used again (in the case of deleted content). 

The higher the number, the later it was inserted into the database.

You could always end up with imported content that is older, but now has a higher Node/User ID because it was inserted at a later time.

You may want to look to the Relation module, or even Drupal's own menu system to define hierarchies.  This depends completely on your use case of course.