I'm rereading Hosts right now, and it sounds like Jack is in his 30's.
Kate's age, pg. 5 "Nearing middle age - in darn good shape for forty-four, she knew, but still six years older than Jeanette..."
Jack's age, pg. 59 "He'd been the last of three kids: first Tom, Kate two years later, and Jack eight years after her."
So Jack is 36.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.