When fans don’t know a characters age, they usually just give the character the actor’s birthdate.
Given this logic, Dave Sheppard was born in 1969, and John was born in 1967. Well, okay, Joe was born in ‘67, though the Stargate wiki lists his birthday as something different, I think? I wonder where that date came from - some official source?
ANYWAY IF WE GO BY ACTOR’S BIRTHDAYS JOHN IS OLDER THAN DAVE.