Reese Witherspoon can hardly believe that her youngest son, Tennessee, is a teenager. "How is it possible that my youngest boy is 13?!" Witherspoon, 49, wrote via Instagram on Friday, September 26. "He never ceases to amaze me with his joyfulness, his brilliant sense of humor and his endless curiosity. I love you so much, Tenn 💙." Witherspoon uploaded a photo of herself and Tennessee, whom she shares with ex-husband Jim Toth, posing on a balcony overlooking a body of water.