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Well, obviously we have no idea how old the baby who played baby Clark was
It's not obvious. The info could be out there somewhere. I know how old the twin baby actors from the second-to-last ep of ENT were when it was filmed.

I wanted an opinion, because if a date wasn't given (and one was, I think with Clark being assigned a birthdate about three months before) I'd try to guess by the baby's age.

I know kids are different sizes. My cousin was 10 and a half pounds when she was born. I think I was about 17 pounds when I was two.