Is it permissible to pray (make du’aa’) for a long life, or is the length of one's life something that is decreed, so there is no point in praying for it to be made long?
Praise be to Allaah.
There is nothing wrong with that, but it is better to add some further definition to what is asked for, such as saying, “May Allaah grant you a long life of obedience to Allaah”, or “of goodness” or “of that which pleases Allaah.” It is known that du’aa’ does not go against the decree of Allaah; indeed it is part of the decree, like medicines, ruqyah (prayers for healing), and so on.
Any means that do not go against the teachings of Islam are all part of the decree.
The decree of Allaah will be carried out with regard to the sick and the healthy, those for whom du’aa’ is made and those for whom du’aa’ is not made. But Allaah enjoins use of prescribed and permissible means, then He decides whatever He wills. All of that is from the decree of Allaah.
And Allaah is the Source of strength.