Does propecia works better when you are younger like 18,19?

Propecia is prescribed for genetic male pattern hair loss.  Age doesn't matter as much as a diagnosis and treatment plan by a doctor.  In other words it usually does not work for front hair loss.  It can also work great for men in their 20s as well as 70s.

That answer is yes (provide we assume the person has genetic hair loss) and that has been shown many times over. 

Propecia at 18 years of age is more effective in younger men like you. Many young people reverse their hair loss.  Your biggest enemy is the progressive thinning leading to a balding pattern that is not evident at this point in time. The degree of thinning that you are experiencing would suggest that you should see a doctor who most likely will get Bulk Measurements of your hair to determine the degree of the thinning that you are presently having. If the diagnosis is early genetic hair loss (the most common cause of hair loss in young men), then the best treatment would be the drug finasteride which is highly effective in men of your age and may not only stop the hair loss but possibly reverse it. If you are developing a more advanced balding pattern the best test to get will determine the Bulk Measurements of your hair and this will show, over a one-year time frame, how effective the treatment will be. Once you know this information, then you and your doctor need to develop a MASTER PLAN which will define what you need to do over the years to come.