Is it permissible to join a scientific research team that aims to "restore youth" to humans, knowing that the Prophet, peace and blessings be upon him, said: "For every disease there is a cure, except old age (decrepitude)"?

Engaging in worldly sciences that benefit Muslims is an act of worship for which a Muslim is rewarded if he intends it for the sake of Allah. Old age and death have no cure that prevents a person from reaching them, and improving some aspects of old age does not contradict the hadith (definition), because these improvements have a lifespan that will eventually be followed by death. Thus, there is no cure for old age that completely heals from it.