I'm 21 year old male and I weigh only 53kg and im 5ft 3inch is that a normal weight for someone my age?

It's Norma. Your BMI is about 20, which is Normal. If you feel good and your weight has been stable, then you are safe, But would still recommend you see a Doctor for a check up and blood tests,
Genes + environment. Hi. It really comes down to genetics + environmental factors. How tall are your Mum and Da? That reflects a genetic input, an would be used to help assess if your height is other than predicted. Environmental factors cover everything else: nutrition, smoking, disease, etc., etc. So if your height is close to the average height for your Mum & Da, that's predicted; if not, maybe something else.
Yes. Weight is controlled by multiple factors. Genetics, (take a look at your family, but remember it is not destiny), Metabolic (how many calories your body burns), Hormonal (mainly insulin and sexual hormones), Cultural (which weight your culture encourages or "tolerates"), Nutrition (which foods you eat), Activity (how active are you including exercising).