Most of us fall into eating the same thing because it’s convenient. It’s not necessary and you’re only limited by your intelligence, imagination, and cooking skills.
What were you doing from age 34 to 39? You added over 20 lbs of lean body mass and decreased your body fat % while gaining weight.
If you’re going to count calories then you can multiply your body weight by 11 and start there. I’m currently doing that and it allows me to eat 2500 calories a day. I find myself hungry a lot because I have a big appetite but I’m not dying.
Here’s a recent article about how to put together your nutrition plan.