Final answer:
Weight gain and an increase in fatty tissue begin to manifest in middle adulthood, around the 40s to 60s.
Step-by-step explanation:
In middle adulthood, which typically extends from the 40s to the 60s, weight gain and an increase in fatty tissue start to manifest. Both men and women tend to gain weight during this time, with men gaining weight in the abdominal area and women gaining weight in the hips and thighs. This is a natural part of the aging process.