Time dilation

Time Dilation is a concept of space/time covered in the Doom novels, and a side effect of The Artifact (in Doom 3: Resurection of Evil).

Resurrection of Evil
The Artifact causes time dilation for those using it.

Doom Novels
Whenever humans travel in 'faster than light' travel, time dilation occurs, where from perspective of the travelers in the ship will only have travelled a few months (or even a year) beyond the Sol System to another star system. However, several years or even a century has passed around them, by the time they return to Earth (see Doom novels timeline).

Flynn Taggart underwent time dilation several times (first only a few days or months to reach a base at the edge of the solar system, and again for a few months to a year to reach a planet in another system), by the end of his adventures and his return to Earth he is over 170 years old (though he hadn't aged a little over a year), he returned to Earth where over a century of time had passed.

Behind the scenes
Time dilation is a difference in the elapsed time measured by two clocks, either due to them having a velocity relative to each other, or by there being a gravitational potential difference between their locations.

It is not known if time dilation in play with the games in the series. Though it could be an explantion for why over 100 years passed between original Doom series, and Doom 3 (as per the ID multiverse timeline) and if they are the same Doom protaganist he did not 'age much'..