In an official blog post, Square Enix revealed the age and a few additional pieces of information about Final Fantasy XV's Noctis and his three merry companions.

Looks like, maybe surprisingly, Noctis is actually younger than Luna:

Noctis:

Age: 20

Birthday: August 30th

Height: 176 cm

Likes: beating games' high scores

Ignis:

Age: 22

Birthday: February 7th

Height: 183 cm

Likes: devising cookig recipes

Gladio:

Age: 23

Birthday: April 2nd

Height: 198 cm

Likes: Coleman's camping (Editor's Note: Coleman is a real world camping equipment brand that appeears in the game.)

Prompto:

Age: 20

Birthday: October 25th

Height: 173 cm

Likes: photography