User blog comment:Nomimushi/Discussion: Shizuo's Age./@comment-33442125-20171110012308

The translation for that gaiden is accurate, but I'm not quite following the issue. If you mean his age at the start of the series his age and Izaya's are correct. This is because it's the beginning of the school year in April. So Shizuo recently turned 23 a few months before while Izaya would turn 24 next month. They are not exactly 12 months apart so in the beginning of the school years their ages are the same.