Is it okay for a guy to be thin as a teenager?

You yourself choose what you want to be! If you want replenishment, you want to pump up. Everything depends only on you. There are no recognized foundations, so ...) the choice is yours!

Normal, and not only in adolescence.

"Normal", or correct, can be considered a physique or asthenic, or normosthenic, or hypersthenic. It is easy to find reliable information about all this on the Internet. Dysplastic physique is considered inharmonious, although this most often does not affect physical health.

The percentage of subcutaneous fat in relation to body weight is a rather variable value and is not always rigidly tied to physique. You can be a complete asthenic or a thin hypersthenic, although this is a rarity. Many teenagers worry about fat or lack of it, but then it goes away. Thin people are less likely to suffer from such massive diseases as hypertension and diabetes. Thinness is in fashion, you must admit that overweight people are often uncomfortable. On the other hand, excessive thinness causes ridicule in the same way as obesity, but this deficiency is eliminated over time, because other qualities in a person become more important to people.

Normal weight can be calculated using the body mass index calculator (beregifiguru.ru).

If you are worried about being too thin, you can start going to the gym, increase the calorie content in your diet (eat more and more often) , normalize the mode of work and rest (sleep at least 7 hours a day).

As a rule, in adolescence, rapid growth is accompanied by thinness, which, one might say, is normal.