Serena Jameka Williams turns 39 years old today. Here are five fun facts about the tennis superstar. 1. She has been playing tennis since she was just 3 years old. 2. Aside from tennis, her favorite sport to watch is gymnastics. 3. The "Serena Slam" tennis term is used when someone wins all four Grand Slams in a row. 4. Williams has served over 120 mph. 5. She has won 23 Grand Slams. Happy Birthday, Serena Williams!