Well i am 22 yers old and i can squat 600lbs and leg press 1260+ easily. Let me tell you that legs need to be done intelligently. I have been told absolute BS by way to many people, even olympic athletes and powerlifters. Funny thing: i squat more than them now!
My workout looks like this. For 8 weeks i will try to get as many reps as possible (until i hit 20) with a certain weight. Right now i am trying to get 405 x 20. So you may do 12 reps first set, then 8. Make sure you push yourself. The key here is to get your joints and body strong enough.
The next 8 weeks i do powerlifting style. Warm-up. Then 405 x 4, 455 x 3, 500 x 2, 550 x 2, 585 x 2 (for examle) on these days i will do squats then 2 sets of straight leg deadlifts. THATS IT! You want your legs to grow then you gotta get strong. On the first 8 weeks i will do squats, then 2 setslunges, 2 sets straight leg deadlift.
As far as gainign weight goes. 3 years ago i was 6'5 and 170lbs. I know how you feel. Now i am 250lbs, 8% BF. I have talked to many people who tell me they cant gain weight and they eat "so much"
Then i tell them i have 14+ scoops of protein a day, 12L of water or more, and eat like 8-10 times... then they are like :O
Maybe i will be a broken record:
But preworkout = 2 scoops protein, 50 g dextrose
Post= 2 scoops, 50g dextrose
1/2 hour later: 2 scoops, 30g dextrose
1 hour late: 1 scoop, small meal
1 hour later: 2 scoops, high glycemic carb meal
If you are not doing something like this you are missing you anabolic window and will never gain weight.
Oh check out AST Sports Science - High-Performance Sports Nutrition Supplements their Max-OT training program is the best ever.