If ya don't know, it is mentioned in the Manga that Kenta is in 5th grade at the beginning of Fusion. Maybe 11 years old. Besides Ryuga's body being over 100 years
Kenta | Wiki | Beyblade Amino
Metal Fight Beyblade - Kenta Yumiya by KinakoJurai on DeviantArt
MFB Zero G ): Kenta Yumiya by LukaKishatu on DeviantArt