demon lords do indeed have 300 years lifespan as you said. But i can tell you there is quite alot of ways to cheat the age limit because of course there is....
( the god who made them loves watching people find creative ways to break all of his rules and only fixes them afterwards)
the age limit is mostly a plot point and a way to determine how much time the other demon lords have had to establish themselfs.
The story is going to be mostly around:
Dungeon/city building and army battles.
( not ONLY for the MC but also for some other dungeon lords and a little bit for normal humans too later on, but 95 % is about the MC)
Cute girls and awesome dudes going cute and/or badass things.
( not ALL monster who are important is a loli/cute girl.... only like half ...so there is some male monster who are important, his next monster for example is going to be a male. )
personal interaction, development and intrigue between dungeon masters,dungeon monsters and a limited number of humans like kings and heroes.
( Dungeon masters are not the only important characters to the story, there are some "wild" monsters with some importance as well as humans, so its not only about dungeon masters ) 
This is also one of the reasons normal humans dont go Apeshit over the more casual monsters like elfs and so on because there are wild monsters in the world who dont work for dungeon masters, and some of them are considered intelligent races like elfs and dwarfs.