I am a man aged 24, l started losing hair at 17, what is the cause?

Hair loss can be seen because o many reasons, a couple of those are;
İt can be hormonal, genetic related  or stress related issues. 

The most common cause of hair loss in men is genetic. Neither sex nor masturbation causes balding.Men's testicles make testosterone and this is metabolized into DHT, a male hormone 40 times as powerful as testosterone. When a person has genes for balding, the hormone DHT works with the gene to cause balding. 

There are many potential causes for hair loss, including hormones, genetics, autoimmune causes, vitamin deficiencies, etc.  It is best to see a specialist who can obtain a full history, possible blood work, and evaluate your pattern of loss to be better able to answer your question and recommend treatment.
Good luck!