According to the Beyblade Wiki, Shu was 11 years old at the beginning of the series (which is true, as stated in the Manga), but the Wiki also states that he's still 11, ONE YEAR LATER during Beyblade Burst God/Evolution, and 2 years later he's 13, which makes NO SENSE and the Wiki is completly ignoring the official timeline in this case.
Alright, so here's my theory.
Shu's official Birthday is September 23rd. At the beginning of Beyblade Burst (Spring 2017) he was 11 years old, according to the manga. During the end of the first season (Fall 2017) he probably turned 12. At the beginning of Beyblade Burst God (Spring 2018) when he became Red Eye, he was still 12 years old. At the end of the World Championship (Fall 2018), he turned 13 years old. 2 years later in Beyblade Burst ChoZ he's 15 years old.
PS: Please note that the years are only place holders and I've put them in there so that they line up with the real world events and WBBA tournament age regulations.
Alright, so here's my theory.
Shu's official Birthday is September 23rd. At the beginning of Beyblade Burst (Spring 2017) he was 11 years old, according to the manga. During the end of the first season (Fall 2017) he probably turned 12. At the beginning of Beyblade Burst God (Spring 2018) when he became Red Eye, he was still 12 years old. At the end of the World Championship (Fall 2018), he turned 13 years old. 2 years later in Beyblade Burst ChoZ he's 15 years old.
PS: Please note that the years are only place holders and I've put them in there so that they line up with the real world events and WBBA tournament age regulations.