Acts 4:21

And they, when they had further threatened them, let them go, finding nothing how they might punish them, because of the people; for all men glorified God for that which was done. For the man was more than forty years old, on whom this miracle of healing was wrought.

Further threatened them ... These were not idle threats. Later, the apostles were arrested and beaten (Acts 5:17-40); and still later, Stephen was stoned to death for preaching the gospel (Acts 6:8-7:60). There is a progression in this inspired history toward that murderous fury which at last signaled official Israel's total rejection of Jesus Christ. For the moment, the popularity of the apostles with the people prevented all but the threats.

Forty years old ... Luke added this bit of information regarding the age of the man who was healed, making the marvel of the miracle all the greater.