My wife is an evolutionary anthropologist and she says that you set your baseline for life under the age of 5. Things like metabolism, reproductive strategies, etc.

There is this great study of Indian girls adopted from an orphanage into Western families. After being adopted, the girls reached menarche at the ages of 7 or 8. The evolutionary signals are clear: if you are starving and then get a huge influx of calories, your body interprets this as temporary so you better reproduce while the going is good.