After an unusually short pregnancy, Karen Powell (Diane Lane) gives birth to a baby boy whose body ages much faster than the normal rate. With her husband, Brian (Brian Kerwin), Karen raises their son, Jack (Robin Williams), who appears to be a 40-year-old man when he is only 10. Though Jack ages rapidly physically, he is mentally and emotionally still a young boy, which causes him considerable difficulty. Eventually, Jack gains the support of his peers, which helps him live a fuller life. Wikipedia