How old was David when Absalom rebelled?

2 Samuel 5:4 says:

4David was thirty years old when he became king, and he reigned forty years.

Thus, he died at age seventy. The Reese Chronological Bible dates David's death to 985 BC and Absalom's rebellion to 993 BC. Therefore, David must have been around 62. Pretty old to be fighting a young buck.