I can understand your confusion; I've often asked myself the same question when hearing of the death of someone whom God was using. I've sometimes wondered if Mary asked the same question when her Son was crucified while still in his 30s.
The same question arises whenever someone dies in the prime of life (or before) -- when a teenager dies in a car accident, for example, or a young mother is taken by cancer. In all honesty, we don't fully know why God allows such tragedies to take place; some day in heaven we will, but not now. As the Apostle Paul wrote, "Now I know in part; then I shall know fully, even as I am fully known" (1 Corinthians 13:12).
What we do know, however, is that even in the midst of tragedy, God still loves us, and we can turn to Him for the strength and comfort we need. Just a few weeks ago, we celebrated Easter, which reminds us of Christ's victory over the powers of evil and death. Because of His resurrection, we know that death does not have the final word; God does. And because of Jesus Christ we know we can turn to God and find comfort and hope in Him.
Pray for this person's family, and in the months and years ahead let them know they are not forgotten. And pray, too, that others -- including even young people in your church -- may heed God's call to carry on this faithful servant's work.