Tank size and what is classified as a juvenile corn?

Hi y'all,

I can't believe it's almost been a year since getting my corn snake. I believe he is already about 9 months old (bought him at an expo in late September, so give or take 3 weeks old when I bought him). What age would he be considered a juvenile? Also, I just measured him, and he's about 22 inches long, which is about as long as the diagonal length in his enclosure. I have him in a 10 gallon enclosure. I did research a while back that once the length of your corn matches or is more than the length of the tank diagonally, then it's time to upgrade. I just wanted to get y'all's thoughts and advice on this topic. I apologize in advance if this has already been answered or in an existing thread. Thank you?