What people tend to forget about infants and small children is that they are constantly exercising, eating a ton of calories, and then getting tons of sleep.

Adults, by contrast, tend to be very sedentary with slower metabolism and a ton of anxiety from stimulants/stress that prevent long, regular rest cycles.

Also, incidentally, if you really want to fuck up a child early in life... Malnutrition, immobility, anxiety, irregular sleep cycles, and lots of stimulants (particularly cigarettes) will have your teenagers looking geriatric.