The finale script page, published by EW, reveals that a 28-year-old Jack first met Rebecca in 1972, at the insistence of the meddling but well-meaning Mrs. Peabody. We know from the show's pilot that the kids were born in 1980, making Jack (and presumably Rebecca?) 36 at the time. That makes sense; the show opened with the kids' 36th birthday, and this means it would have showed Jack's 36th birthday opposite that. If Rebecca were also 36, it's realistic that she had complications with her pregnancy, too, which sadly led to the loss of the couple's third triplet.