Could you tell me if my hairline is receding at 21 or if I have a mature hairline? (Photos)

yes It is, I would consider hair transplant to at least the hair line and starting NOW ASAP prp/acell injetions in the office, at home growth factors and roller protocol weekly + rogain (see link to emeragecosmetics.com). we do virtual consults.I suggest a formal consultation as there are a combination of treatments that can be done to improve har loss.  Oral finesteride has very little to no side effects in the majority of people; PRP/platelet-rich plasma + stem cells like A Cell help regrow hair and slow down the thinning process; rogaine and restorsea spray should be used daily; oral viviscal helps keep nutrients high in the follices as well.  Transplantation is a last resort but works early in the aging process as well.  A formal consultation and combination treatment is needed.  Best, Dr. Emer. 

Some ppl have mature hairline since birth ,plz compare your existing pictures with previous pictures if forehead appears broader and hairline has receded also if there is existing hairloss in the other areas,then it is a case of male pattern baldness ,it is suggested to try with medical hair loss treatments initially to arrest the progression and reduce hairloss. Hair transplant is too early to opt as of now .Please visit dermatologist in your city to know details about medicines. 

To define a mature hairline, I must see the eyebrows lifted high so that the forehead creases. A Mature Hairline is located one finger breadth above the highest crease in the middle.

If you r hair line was lower and is now much higher than before May be you are receding. Some receding as you get older is normal. That is the reason we do not jump into hair transplant procedures at young age. If your receding continues and also leads to hair loss in other parts of your scalp, in that case you may be having genetic male pattern hair loss. If you are concerned, it could be a good idea to see a hair transplant specialist.