I'm 58 and gained 100lbs in the last 4 years. I did it by eating a lot of clean food, but by adding in good fats and trying to eat calorie dense foods. Also as far as supplementation when you hit a sticking point add 8 weeks of an oral like t-bol, then come off the oral and go to peps, ghrp6, ghrp2, ipam, or hex, my preference is the ghrp2&6 along with cjc1295 dac.
I cut my training regimen way back to a power lifters routine where I do only compound movements. Flat bench, squats, deads, military press, bent over rows, curls and tricep extensions. I warm up thoroughly since I'm really old, then do only 5 working sets of the movement I'm on for that day. Now legs I will add in leg presses, extensions and curls, but only according to how I feel that day because 32" quads need a lot of maintenance. For example on arm day I superset curls and lying tricep extensions. I warm up to 135 or 155 depending on my elbows, then do 5 sets. I don't worry about the tricep being too light I just rep it 15-20 reps because I get plenty of tricep work doing bench press, with a working weight of 10 reps at 455-475. I had gotten up to 495 for 10 reps 3 sets, but then had a neck injury. And keep everything in thet 10-12 rep range unless blowing it out with super drop sets. But at my age I have to do these on rare occassions when I actually have that much energy.
I hope that helps some.
Also one BIG thing I forgot to mention is lay off the protein shakes. Use real food!!!! I cannot stress that enough.