I'm 46 now and all I do is exercise to maintain where I'm at. When I was in my early 20s I started a bodybuilding routine and did this on and off into my mid to late thirties. I gained from 155 up to 205 lbs during that time. This was all natural and the only supplements I took were a carb drink and amino acids.
Assuming you are using a bodybuilding split routine, you are using enough weight, and you are eating enough protein, usually the problem with gaining weight isn't the amount of protein you're eating. You should eat a lot of protein but bodybuilding workouts burn mainly carbohydrates. A good weight gain diet is approximately 55 percent carbs, 35 percent protein, and 10 percent fat. The fats you eat should mainly be non-saturated fats like nuts. The carbs should be whole-wheat breads, vegetables, cereals, fruits, and even some pasta. If you don't eat enough carbs your body will resort to using the proteins for energy. If this happens it takes away from the protein you have available for muscle growth. Also remember that, scientifically, for the average human a muscle gain of 10 lbs a year is maximum. There are exceptions, but people who claim to gain 30 or 40 lbs of muscle in a year are usually gaining fat in addition to the muscle, or they're taking steroids. Most people will gain some fat with their muscle mass and this is why bodybuilders cut their diets and do carb loading in the last phase before a contest so they will lose the fat and get cut. So I would suggest making sure you are eating enough carbohydrates. What I always did was drink a carb drink that provided about 80 grams of complex carbs one hour before working out. I found that I had more energy, was able to lift heavier weights, and was able to do more reps when I did this.