You are far from being too old to learn to box, but you should bear in mind that, should you come to decide to pursue the activity seriously- as a pro- that you have no time to waste, like those guys that started at eleven and got to play for a few years
If you want to box start now; don't waste time trying to get "fit and built up" because you have, as of yet, no real idea of how fit you need to be or if building up would work to your advantage. Start a regimen of exercise that features 15 minutes or so of stretching exercises and calisthenics, as well as as fast paced early morning walk of 1-2 miles. After you finish at the gym in the evening you'll want to do some "roadwork"; I'd recommend finding a jogging track and the following program: break your work into three 3 minute rounds with a 1 minute rest period between them. Before you begin walk two rapid laps to warm up, and then go directly into the first 'round'. You'll want to alternately jog at a good pace, run, and sprint, breaking up the three minutes @ evenly. During the one minute rests continue to walk briskly. You'll treat the other two rounds as you did the first, but trychanging the pattern so that in one round you run for the majority of time and in another round let sprintimng carry the day.
As you work on your punches when you first go to the gym, do not neglect your defensive skills. As you learn each punch in turn learn the basic defensive moves to protect against the punch. You'll be thankful when you do spar, which should be within 3-5 weeks; by that time you'll be able to do a couiple rounds and you'll be ready to learn about how to actually land punches