Adding muscle in your 70's is not likely, but regular training can keep a guy in great shape. I saw the one and only Jack LaLaine in his 80's in amazing shape - he did a hand stand like it was nothing. He just kept at it. Frank Zane kept himself in great shape too.
+1 However, weight training is good for the body, (but what you use to do as a young man is not necessary or recommended) and not just muscle tone, but exercise lowers your risk of heart attack, can help to improve your blood sugar and control weight gain if done right and in moderation. Using repetitious moderate weights or machine type moderate weights can help you burn calories and keep your muscles ready for when you need to actually use them. Even if you can still handle the heavy weights in your latter years, and you may not feel that you're straining, it's not good for your joints either. Moderate free weight training, some form of cardio exercise and keeping an eye on what and how much you eat should keep you healthy. If you can't do any of that, at least walk every day or every other day. Do whatever you can to stay healthy. jmo