There is no age too old or too young to work out, and work out is a VERY broad term. There are SOOOO many types of specific work outs out there, if you do enough research and reading, you'll find there are workouts specific to cutting weight and toning muscles, as well as workouts intended to build lean muscle.
I am no expert so don't take what I say as golden, but I have done a lot of "reading between the lines" over the last month as I started my journey to get healthy and lose weight. The first thing I would do, is figure out your BMR (Basal Metobolic Rate) just google that and read read read. The fitday program here will calculate this automatically for you as your base "calories burned" before activites. If you are looking to gain weight, you need to take in more calories than you burn in a day. Not only do you need to take in extra calories but you need to take in SMART calories. So the next thing you need to google is "eating clean" and read up on some suggestions on how to eat clean. Also look up "Protein per pound of body weight" and get an idea of how much protein you should be taking in. From everything I've read, healthy bodies are made in the kitchen (at least 70-80% of success will come from the kitchen)
Last but not least, do a little research on work outs to build muscle. Cardio such as running, aerobics etc... are going to speed up the heart rate a lot and burn calories, if you don't have enough calories in your system from food, then the body will start to cannibalize on your muscle tissue for energy for cardio. This is a bad thing for anyone, but especially someone trying to get bigger (not saying cardio is bad, but cardio without a proper diet will break down muscle tissue as well as fat cells, making your eating habits doubly important). So you might want to focus more on a slower, heavier weight, less reps type of workout/weightlifting program. Don't focus so much on spiking your heart rate, but more on working your muscles hard and heavy. As you work them, they will get micro-tears (this is why you get sore the next day) and those micro tears are rebuilt using protein in your digestive system. If you take in the right amount of protein, and the right balance of types of proteins, then you will see muscles build and get larger, as well as have much faster recovery from the soreness aka micro-tears.
I don't look at cardio so much as a fat burner, or muscle builder, but more of a lung/organ builder. Cardio strengthens your insides, weight training strengthens everything that people see. Oh and BIG thing about weight training, don't just train your upper body, and NEVER mistake cardio such as running as a "leg" workout. You need to well rounded workout plan that includes things such as Lunges, squats, and deadlifts to strengthen your legs. Good luck and stay determined!