Goku is 11 when they first introduce him in the first episode of Dragonball. Then by the 21st world martial arts tournament he turns 12. Then after that he defeats the red ribbon army, and then trains for 3 years, and by the 22nd world martial arts tournament he's now 15. Then after the tournament he defeats King Piccolo and then trains with kami and popo for three more years, so by the 23rd world martial arts tournament he's now 18. Then at the start of dragonball Z - The Saiyan Saga - 5 years have passed since Goku defeated Piccolo Jr. at the 23rd World Martial Arts Tournament, thus making Goku 23 at the beginning of Dragonball Z. I hope this helps.