I'm not sure, not having read the paper, but I would expect that "Lifetime manic features at age 22-23 years" means "number of manic features experienced in the time prior to 22-23 years of age" (i.e. we measured IQ of a bunch of 8-year-olds 15 years ago, and those people are now in the range of 22-23 years of age, and we ask how many manic episodes they've had in that time).
Ah, that makes sense.
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