Arlo is actually younger. He is a second-year king (said Rei) and Everyone else is as old as John and Sera and they are third-years (John at New-Bostin, the skipped year, now this) so I think hes younger.
@Bekah137 I have to disagree on one thing you said. He was not absorbing Arlo, Meili and Ventus's abilities but actually copying them. Otherwise your response is great.