If your goals are to get stronger and put on some muscle mass - then you have to eat to support these goals. My general advice is to follow a basic template (protein (meat/eggs), fruit/vegetables and then make up the difference with carbs) at EACH meal. The good thing is that once you develop this habit, everything becomes easy. You simply adjust the portions to fit your goals.
So each day I have a goal of how much meat to eat. I space that out during the day. When I was trying to get bigger, I would have a "meat and egg goal". For example, 2 pounds of ground beef and a dozen eggs. I'd eat those goals throughout the day and then have an apple or whatever with each meal and then fill in the rest with whatever I wanted. If I was too heavy, it would be rice. If I wanted to bump up, it'd be something calorie dense (fruit pie).
What you end up doing is making a life-long habit that can be easily adjusted to fit your goals and your schedule.
Obviously an older person with a slower metabolism has to eat less (or someone without 25 years of consistent training every single day aka Sweat Equity) so you just take the same habits and adjust them.
That is why diets suck and habits rule - you can take them with you for your entire life and simply adjust them however you want. So find YOUR diet habits if they aren't in-line with mine and pound them into your life.
I would also advise anyone to do SOMETHING structured with training every single day. For an older person, I would advise taking 10-20 minutes every day to do some mobility/flexibility work and perhaps 20 minutes of "easy" cardio. "Physical" is something that is part of my daily ritual and helps, again, to develop solid habits. Doesn't have to be hard but it should be something.