Is it permissible to pray for a long life when one's lifespan is already predestined?

There is no harm in supplicating for someone to have a long life. However, it is preferable to qualify it with what benefits the person being supplicated for, such as saying: "May Allah prolong your life in obedience to Him." Supplication does not contradict divine decree (Qadar); rather, it is a part of it, similar to permissible means such as medicines and ruqyah (incantations). All of these are part of Allah's decree, Who has commanded us to utilize means.