In Solo Leveling (both the manhwa and novel), Sung Jin-Woo starts the story as a 17-year-old E-rank hunter at the beginning of his dungeon-crawling journey. However, his biological age becomes tricky due to:
Time Dilation in the System: His training in the "Instant Dungeon" means he experiences years of combat in compressed time.
Post-Revival Adjustments: After his rebirth as the "Shadow Monarch," his body is enhanced beyond normal aging.
By the end of the series, he’s physically in his early 20s, but his actual lived experience (including time as the Monarch’s vessel) could be argued as centuries.