When is Yu's Birthday ?

Yuu has no canon birthday.
It's possible that he has the same horoscope sign as his beyblade - Libra (Just like Kyouya has the same sign as Leone), which would mean Yuu's birthday is between 23rd of September to 22nd of October.

Yuu is about 10-11 years old when he first appears, so it's very possible that Takafumi Adachi thought of the funny idea to make a character 10 years old and born on the 10th دن in the 10th ماہ (10th of October, which would make him a Libra just like his beyblade) and name him "Yuu", which is Japanese for "Ten".