The 238-hour private pilot and three passengers attempted to depart the 1,900-foot private grass airstrip in a vintage 125 horsepower single-engine airplane. The pilot reported that he elected to depart the airstrip in a northerly heading to take advantage of the very light headwind; however, he was aware of the tall trees at the departure end of the runway. During the takeoff roll, the pilot added that he extended the flaps to the first notch as the airplane accelerated through 60 mph. The pilot added that he was able to rotate at 60 mph, and the airplane appeared to initiate a climb. Shortly after takeoff, the airplane appeared to have encountered some "sinking air" and the airplane started to settle into the tall trees located near the departure end of the airstrip. The 1951-vintage fabric covered airplane was not equipped with a stall warning indicator. There was no post-crash fire and the pilot and passengers were able to egress the airplane unassisted. The pilot reported that there were no anomalies with the airplane's flight controls or engine. The pilot further reported that he should have "stepped off" the field length and calculated the density altitude on paper instead of figuring it in his head. The weather observation facility located about seven nautical miles southeast from the site of the accident was reporting calm winds. The field elevation at the airport was approximately 1,073 feet and the density altitude was calculated at 3,122 feet. The estimated weight of the airplane at the time of departure was near its maximum takeoff weight of 1,800 pounds.