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Frosty January!

11-year-old actor Jared Gilmore played Bobby Draper -- the son of Don (Jon Hamm) and Betty (January Jones) -- on the first four seasons of Mad Men.

But the actor is moving on to star on ABC's Once Upon a Time, premiering this fall. Whomever replaces him as Bobby for the AMC hit's next season should beware, Gilmore warned.

"Be careful around January Jones," Gilmore told TV Guide of the 33-year-old actress, whose Mad Men character is famously cold and closed-off emotionally. "She's not as approachable as the others. She's really serious about what she does. Everyone else is so nice."

Jones is currently pregnant with her first child, due later this year. She has yet to reveal who the father is.