I'm 69 and still get my heart rate over 170 on a tough climb. I came back to cycling four years ago after almost eight years off due to my worn out knees. With new knees I ride slopes as steep as 12%. Power to weight ratio makes a big difference. I now weigh 133-134 which is about 8 pounds less than when I came back. It took over two years and 10,000 miles to get my fitness back. One important thing to get back was my ability to pedal standing for up to a mile to get up some of the steepest slopes. Pedaling out of the saddle allows me to use about two sprockets smaller than pedaling seated.
I found a modern bike setup to be valuable. I was riding a shimano grx 46/30 crank with sram force axs 12 speed 10-36 cassette. I'm using a 48/31 with a 10-33 cassette now that I'm in better shape.