This is my head canon on Yusuke's aging, since it's not really specified in the manga.
T.O.D. (Time of Death; first chapter) Spirit Detective Artifact Recovery Arc Rando Arc Saint Beasts Arc Rescuing Yukina Arc
---->Age: 14*
Dark Tournament
---->Age: 15
Sensui Seven Arc Three Kings Arc
---->Age: 16
Final Mission
---->Age: 18*
*Note: It is stated that at the end of the manga, for the final mission, he's 18. He also starts the series at 14. These two ages are set. It's merely the timeframe in between that is my head canon.
Aging For Yusuke | Apps Reference
T.O.D. (Time of Death; first chapter)
Spirit Detective
Artifact Recovery Arc
Rando Arc
Saint Beasts Arc
Rescuing Yukina Arc
---->Age: 14*
Dark Tournament
---->Age: 15
Sensui Seven Arc
Three Kings Arc
---->Age: 16
Final Mission
---->Age: 18*
*Note: It is stated that at the end of the manga, for the final mission, he's 18. He also starts the series at 14. These two ages are set. It's merely the timeframe in between that is my head canon.