You are certainly not too old. I started at 41 and am now 44. I train and spar with the competing amateurs in my gym. When I'm in the gym I forget about the 20-30 year age difference and the other boxers think nothing of my age. Make sure that you are in good physical condition before starting training in your boxing gym e.g. make sure that you can do around 50 repetitions of push-ups, squat thrusts, 'knees-to-chest', squats, lunges and around 100 sit-ups. In our gym we would often do something like this at the end of a session involving around 3-4 rounds of sparring and a few rounds of pads and bag work.