Do I have a mature hairline or am I balding at 21? (Photos)

The first question I would ask is whether or not you are noticing more hairs falling out than normally.  It is hard to tell from your pictures if your hairline is receding.  You have said you have noticed that it has receded.  It would be best for you to be evaluated by a hair transplant surgeon.  If it is receding though, it would be wise for you to start measures to prevent further hair loss.  Topical minoxidil (Rogaine) is one option.  This can be obtained over the counter and should be used once a day to the affected areas.  Other options for treatment include finasteride, Platelet Rich Plasma (PRP) injections and the use of a laser cap.
Finasteride is a medication that you take once a day.  It works by blocking an enzyme that converts testosterone to dihydrotestosterone (DHT).  DHT is a compound that promotes hair loss at the level of the follicle in those who are genetically susceptible.  Thus by using finasteride less DHT is produced and less hair loss occurs.  In your case it is safe to use.  The side effects most talked about are sexual side effects (loss of libido, etc).  However this occurs in only about two percent of individuals who take the drug.  And if necessary, once stopped the side effects resolve.
PRP injections are another method used to prevent hair loss.  A small amount of blood is collected and spun down to concentrate the platelets in a specific layer.  The platelets are then injected into the effected area.  Platelets contain "growth factors" which all cells use to promote growth.
The laser cap emits light of a specific wavelength that is absorbed by the cell to promote its growth also.  
While all of these methods help prevent hair loss in some people, especially those who are younger, new hair growth and thickening of current hair can occur.  

With  bitemporal recession, it appears that you have male pattern hair loss. This would need to be confirmed by your hair loss expert. Examination of the other parts of your scalp and a microscopic evaluation should be performed. Finasteride is a drug that should be discussed with your physician. Find an expert. 
Jeff Rapaport MDRealself Top 100

I would recommend getting a microscopic evaluation by a hair transplant surgeon to better understand medical vs surgical options. 

Your hairline is no longer straight across.  Most men show thinning and receding at the front and sides of the scalp in their twenties and thirties.  You may be starting a little earlier.  The good news is that there are excellent treatments available for you.  Find a board certified dermatologist who specializes in the treatment of hair loss and you should do fine.  

I suspect you have more than just maturation of the hairline. Please speak with a physician for options.

From the pictures it would appear you have a receding hairline. Very typical of male pattern baldness.

It does look like your hairline shape is receding. To make sure, take a pictures with your eyebrows lifted high and a mature hairline is one fine breadth above the highest crease