Jesus tells us not to worry. (Matthew 6:25-27)
As for prayer, whatever happens is God’s Will. God does already have factored into how things go in the world based on prayer, works, charity, of all humans. If you look at Philippians 3:19, and read into it, it’s saying we should not be worldly. Set your mind on Heavenly things.
If there’s a chance in the future of a person turning to God (or through their good/evil actions, turning others to God), God (likely) will grant that person a longer life. God works all things for the good of those who love him. (Romans 8:28)…Since God isn’t subject to space-time the way we are, those who love Him would be known to Him by the completion of their life.
So realize your prayers may help more if the means to an end is conversion and contrition of self/others (through good or evil action).
There’s no foolproof method of avoiding illness with prayer.
So to answer: yes/no/maybe. We can’t change God’s mind, but God knows all that you will ever do and has factored that in.