Then the men of Israel said to Gideon, Rule you over us, both you, and your son, and your son's son also; for you have saved us out of the hand of Midian.
"And the land had rest forty years in the days of Gideon"  (Judges 8:28).  This indicates a rather long life for Gideon, and it was quite logical to append at this point in the narrative a kind of summary of that long life.