Your never too old to start training.

I was a skinny runt when I was yound and I found that through my late 20's my metabolism slowed up and I started to be able to gain a little size. You may find that when you go back to the gym things come a little easier this time? Who knows until you give it a try. Eitherway, as already said, you have to up your carb intake. I would also say that instead of a protein shake you should try a "heavy weight gain" shake. These contain protein but lots of carbs too whereas most protein shakes try to limit the carbs.