Board Thread:New on Ben 10 Wiki/@comment-24579099-20140701182447/@comment-24770490-20141013190858

Blaziken rjcf wrote:

No. We saw Kevin at age 11 and 12. He's one year older than Ben and Gwen, and Ben and Gwen were never shown as 12 year-olds.

Pretty sure he meant the Kevin flashback from Many Happy Returns but he was "obviously" older than 13.