Is there a specific lifespan for a human that cannot be exceeded? Does life lengthen or shorten depending on the righteousness or corruption of deeds? Does neglecting the body and refraining from medicine shorten one's life? If life is predetermined, why do we take medicine?

The appointed term (ajal) and death are part of Allah's decree and destiny, written in the Preserved Tablet, and cannot be changed. Allah the Almighty says: (And Allah will never delay a soul when its appointed time has come.) [Al-Munafiqun/11].

However, this does not mean that they are not subject to the law of causality. Death has material and spiritual causes that can delay it, such as maintaining health, avoiding danger, supplication (du'a), upholding kinship ties (silat al-rahim), and honoring parents. Narrated by Anas (may Allah be pleased with him), the Prophet (peace be upon him) said: (Whoever wishes his provision to be expanded for him or his lifespan to be extended, let him maintain ties of kinship.) Narrated by Al-Bukhari and Muslim. And narrated by Salman (may Allah be pleased with him), the Prophet (peace be upon him) said: (Nothing averts the Divine decree except supplication, and nothing increases life except righteousness.) Narrated by At-Tirmidhi.

All of this is known and written with Allah the Almighty, from its beginning to its end. And His saying, the Almighty: (And no one whose life is lengthened is lengthened but it is in a register, and no one whose life is shortened but it is in a register. Indeed, that is easy for Allah.) [Fatir/11], means that Allah knew what the outcome would be and recorded it.

Thus, seeking treatment is one of the material means that preserve life and health. If a person's days are prolonged, it is by Allah's decree, and if they are shortened, or if he abandons treatment until his appointed time, that is also by Allah's decree. Narrated by Abu Khuzama from his father, that the Prophet (peace be upon him) was asked about ruqya (incantations) and medicines, whether they avert anything from Allah's decree, and he replied: (They are part of Allah's decree.) Narrated by At-Tirmidhi and Ibn Majah.