Don't worry, you'll gain the weight with age! haha. My best friend is about 3 years older than I, and he's been underweight all his life. Unfortunately, when he hit 25, things started changing....and staying naturally fit was a little harder than it use to be for him. He was still slim (roughly 98 lbs), but it was more fat than anything. Long story short, he decided to get a gym membership and got an aggressive training buddy that helped him pack on about 20 lbs. (Not all muscle, but most of it is) He's bigger and stronger than ever now! His secret? Hard work, protein powder, and creatine.
As for myself. I'm not a big fan of creatine. Protein supplements are good though. Make sure you take one after a workout if you don't have time to eat, and also try taking a shake when you wake up and when you sleep. That might be excessive for some, but for us ectomorphs, we need it! Only problem....protein can get kinda pricey. When I found out I was lactose-intolerant...it got even more expensive because I couldn't use regular Whey protein.
As for tips while you don't have equipment or a gym membership. Push ups, sit ups, and pull ups (get a pull up bar....shouldn't be too bad).
Also check out the link below. It's Men's Health's outdoor workout and its free for a limited time.https://shop.menshealth.com/downloads/outdoor-workout.html