Questions & Answers about Die Bevölkerung in der Stadt ist jung.
The preposition in is a Wechselpräposition (two-way preposition).
• When it indicates location (answers “Where?”), it takes the dative case.
• When it indicates direction or movement (answers “Where to?”), it takes the accusative.
Here we describe a location (“in the city”), so we use dative. Stadt is feminine, and the feminine dative article is der, giving in der Stadt.
German commas separate clauses (especially subordinate or relative clauses) or list items.
This sentence is a single main clause: subject (Die Bevölkerung in der Stadt), verb (ist), predicate (jung). There’s no subordinate clause or enumeration, so no comma is needed.