John Thayer Jr. (Jack) was just seventeen years old in April of 1912, and was traveling in first class on the Titanic with his parents, John Borland Thayer (president of the Pennsylvania Railroad) and Marian Thayer, along with his mother's personal maid.
Jack was just getting into bed on April 14, 1912 when he noticed the breeze through his window slow and then stop. He told his parents he was going out to "see the fun" and threw on an overcoat. He went outside to see others kicking ice about on the decks.
Through all the chaos he lost track of his parents, and ended up with a shipboard acquaintance, Milton Long. At 17, Jack was considered a man, and not eligible for a seat in a lifeboat. He and Milton stood on the starboard side, looking over the railing. As they watched the crowd, and the water creeping closer to them, they decided the time had come to jump. They shook hands, and exchanged messages for those back home. Milton looked at Jack as he took off his overcoat and said "You coming boy?"
"Go ahead," Jack replied. "I'll be right with you." Milton slid down the side of the ship. Jack decided to jump away from the ship as far as he could. When he surfaced, Milton was gone. He never saw him again.
Jack was able to swim his way over Collapsable B, which was floating upside-down in the water. There he balanced delicately on the hull with many others all night long, until another lifeboat finally came to their rescue the next morning. They were then picked up with the other survivors by the Carpathia, and taken to New York.
Jack Thayer wrote a very good account of his experience that night. Sinking of the Titanic is available through the Titanic Historical Society.
Jack's mother and her maid both survived the sinking, escaping in boat 4. His father did not survive, and his body was never found. Jack Thayer died in 1945.
Special thanks to Encyclopedia Titanica, and Unsinkable: The Full Story Of The RMS Titanic by Daniel Allen Butler for helping me fill in all the facts.