The Eternal Life Contract

The Eternal Life Contract (영생계약, Yeongsaeng Gyeyag) is a special Contract created by the Administrator to slow down the aging process of Tower inhabitants.

All people who did not sign or make an eternal life contract, they all age, they become old. It is just pace/speed that differs from a person to person. But once they get into the Inner Tower area, the aging process does slow down. And actually, even if you made an eternal life contract, it doesn't mean you will not become old. Growth just ceases or stop at some point.