He is not a kid.
THE BEAST EMPIRE EXISTED LONG BEFORE, WHEN THE BEAST KING AND QUEEN WAS ATTACKED BY THE EMPIRE AND THEIR MAGES THE QUEEN WAS PREGNANT RIGHT? so she was sold and raised Javi in the slave center,that was way long ago,it was the reason the mage tower was locked with such restrictions so it was long ago before FL was born so even though Javi appears like a young boy because he didn't receive the stimuli to come out of the cocoon of his young self because of his mother's death and malnourished, but now that the beasts are recuperating and regaining strength and he has a purpose in life,which is FL, since she saved him from the slave traders Javi awakened,so Javi is actually older than FL in terms of age.
I shipped so hard for lucas or at least kassion but why that kid. Nah we had something good author what happened