According to a 1997 Sega Saturn magazine interview, the game's planner/director Takashi Oda mentioned that he wanted to target the game towards adults rather than children or teenagers, and did so by making the protagonists a decade older compared to other games at the time which commonly featured protagonists in their 20's, allowing the developers to avoid toning down the violent content in the game for a younger audience:

"I did that because our target audience was adults and up. From the start of the project we knew we didn’t want children playing this game. Had we wanted to target them, I think we would have made the game more about ghosts or something, not zombies. Those considerations were one of the tougher parts of this development."
Contributed by ProtoSnake
Sega has lost the original source code to House of the Dead, meaning they will never be able to re-release this game again without reverse engineering a different version of the game.
Contributed by MrConnman123