Hey I'm 19 years old and have realised that I'm thinning/ balding. What can be the cause at this young age.?

It's called Male Pattern Baldness, and it is hereditary. It usually starts around that age. Mine did.

You can spend gazillions on regrow goos, or the different "clubs" for men, but I just let nature take it's course with mine. I wash, condition, dry and style my head with a washcloth and a towel. Much easier than it used to be.

Bald is in! Chicks love us bald guys.

Could be stress, hormones, lack of sleep, etc. Why not seeing a doctor?

Heredity. It can skip generations as well.
Or, some people are diagnosed with Alopecia, where the hair thins out and falls out, (at any age) continuously. Keep an eye out, for hair in the shower. If you are really concerned, see a Doctor. There are products that will help regrow hair for some people but are very expensive treatments. Hey, bald is beautiful.

Genetics. Unless you wear a helmet on a daily basis. Helmets tend to speed up male pattern baldness.