How can I categorize entites by their age in an entity relational diagram?
This would be a case for either filtering the data during your query (WHERE clause) or creating a view that already filters it out. In normal practice, a person table is a person table, and unless the fundamental fields are different (like the difference between customers and employees), then all person records go into the same table, and the diagrams reflect that.
When drawing things up, start with just how the data is related across the database, then start worrying about sorting, grouping, and filtering based on the data for when you are planning out stored procedures and views.