Am I underweight, if so, why (TW)?

My name is Aylin & I'm 13 years old.

I'm 170 cm tall and weigh 51 kg. My stomach is pretty thin.
I have already told my mother that I find myself too thin & she said, Every girl your age wishes to look like you. "

I'm not taken seriously. I once had everything checked by the doctor. We were able to rule out any kind of deficiency & we could also rule out thyroid problems. My doctor said that everything would be fine with me. He also said that your body is very thin, but top models are just as thin as you. "
I eat 3 meals a day & always eat the plate empty. I also eat a lot of fruit and vegetables every day. I also don't eat sweets or drink sweet drinks. I drink 2-3 liters of water a day.

Can my low weight also be due to a lack of exercise? Because I do nothing but watch Netflix & listen to music.

Can you answer my question

Your weight is still in the normal range and will mainly be explained by your low age. You are not fully grown. In the next 5 years you will also put on weight without changing anything in your lifestyle.

So leave everything as it is - like to move a little more and do sports. Especially now in the time of the exit restrictions.

Eat

According to the BMI, you are actually slightly underweight. Your ideal weight would be between 55 and 60 kg. (https://www.bmi-rechner.net/cgi-bin/bmi.pl) However, this is normal in your age and size, I think. If you are 1.70 at 13, you have probably grown quite a bit and your body needs 1-2 years to "follow up". So it's all green and nothing to worry about. Especially if you eat enough and your doctor tells you that all the necessary nutritional values are covered.