I am 6' and weighed 130lbs from the age of about 17 till around 35. It sucks being skinny, folks always asking if there is something wrong and telling you to eat more.
I started gaining weight when I started doing some strength training (and no cardio). Muscle weighs more than fat, so adding muscle added weight. I boosted my caloric intake using smoothies with protein plus high calorie good for you stuff like almond butter, coconut oil, flax, etc. My challenge was it was so damn expensive consuming a healthy, good for you, 4-5,000 calories a day.
My metabolism has slowed a tad since 36, which is a blessing. IF you are medically healthy, it's probably just one of those things you may have to live with until you get a bit older and your body slows down.