I'm a 19 year old male with a pretty badly receded hairline. Did excessive masturbation cause my hair to thin so early?

Masturbation is a choice, hair loss is not.  This is genetic and not anything you could have done based on your post.  I suggest you see an expert if you are looking for preventative treatments and an evaluation.  Best, Dr. Emer.

hair loss is genetic. the amount of masturbation does not affect your hair loss. since you are young, you should have a consultation with a hair transplant surgeon who can review your scalp, and discuss your nonsurgical options so that you can slow the progression of hair loss.

For most men, the cause of hair loss is androgenic alopecia or “male pattern” hair loss. Dihydrotestosterone (DHT), a breakdown product of Testosterone, is the trigger that causes miniaturization on genetically susceptible hair follicles.   DHT shortens the hair growth cycle, producing shorter, finer hairs. With continued exposure to DHT, without treatment, the genetically susceptible follicles eventually disappear leaving a bald scalp.

It sounds like you are experience male pattern hair loss and it has nothing to do with excessive masturbation.  I recommend you speak with an hair loss expert regarding options on stopping the loss.