Is it normal to be skinny?

You are about the same size I was. When I was 18 in the army I put on 30 lbs of muscle in a single year, That is pretty normal if you work out. Adult men tend to be far stronger and that usually happens somewhere about 20 years old depending on the person but it requires that the adult works out. I worked out hard as a 16 year old. I was on the wrestling team and all that. The body responds much differently when you reach a certain age. I was considered extremely strong in the Army though I didn't grow much taller. I fought as a heavy weight in Navy Aviation Officer candidate boxing match but I was a bit older than most of them at about 28. It wasn't even a fair match with anyone else. I could have destroyed them if I tried to be honest. I injured my opponent in sparring and I didn't even mean to throw a hard punch. Just a flick of the arm jab. I grew stronger than even our heavyweight state wrestling champ and that guy was huge to a high schooler. He is probably bigger than me now.

depends on you

Your genes determine if you are naturally thick (muscular) or thin.

Yours say to be thin. All that you can do is work with what you have.

Lift weights/use strength machines to make the strongest thin muscles you can.

It's normal and millions of fat people would kill to have your body.