i think more than anything, it's all dependant on the quantity of food you eat, you could have the best workout in the universe but if you aren't eating enough calories per day you can't expect to get heavier by packing on muscle, it's like trying to build an extension on a house with not enough bricks. I'd say the biggest factor in getting big is eating enough calories and adequate amounts of protein, what bp2000 is referring to is when your younger you generally have a higher metabolism which means you need to eat even more calories to be in surplus and start gaining muscle, as you get older and it starts slowing your bmr drops and you don't have to eat quite as many calories to be in a surplus. if the scale is not moving upwards (0.5lb a week is fine) and your not getting heavier you can't be gaining muscle, its extremely difficult to both gain muscle and lose fat at the same time effectively balancing weight. You can lose body fat and maintain muscle showing a net decrease in weight, but if your trying to actually gain muscle you need to be getting heavier, if you aren't, eat more!
