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 Yumiya from Beyblade: Metal Fusion
beyblade kenta on Tumblr
Kenta Yumiya | Beyblade Wiki | Fandom
Beyblade: Metal Fusion" It's Our Special Move! Sagittario (TV Episode 2009) - IMDb