I... assume he's somewhere in his twenties, honestly? I wouldn't really know, he seems like a private person that doesn't talk about anything that isn't Kaibacorp or duel monster related in most interviews I've seen on TV.

[Jun has not actually had the opportunity meet Seto Kaiba in person before now, as the Manjoume Group did not exactly want the youngest member of their family meeting the guy until he actually had something worth bragging about, as far as they were concerned.]