Am I a candidate for PRP? I’m 18 year old with receding hair sides. How effective is PRP in long term? (Photos)

In my experience, PRP works very well in young individuals. It can decrease hair loss, increase the thickness of the shaft diameter and regrow hair in many young people. However, It is not possible to predict how PRP will work on a long-term basis. This is dependent upon your genetic predisposition. The goal of PRP in your case should be to keep the follicular units alive. There will be superior hair  regrowth technology available during your lifetime.
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The research performed and the anecdotal evidence would suggest that thickness of hair and density can improve with PRP injections.  The number of sessions as well as the quality of PRP obtained will vary considerably physician to physician.

PRP injections are a great option for young patients with hair loss. There are more studies coming out all the time supporting the use of PRP for hair loss. I do see good results in my patients as well when performed over a series of treatments. As with any hair loss treatment regimen, I do recommend a multi-faceted approach to hair loss, with things like PRP, finasteride, minoxidil, nutritional supplementation, etc. Eventually, hair transplantation may be an option if the hair loss continues, but hopefully with things like PRP, this can be prevented.
The data for PRP's effectiveness over the long-term is lacking, so I typically recommend that patients stay on a maintenance regimen with intermittent injections every 6-12 months after the initial series.
Be sure to see a hair restoration expert to discuss these options in detail.
Best,

Long term PRP data is not there but there is promising data is showing that there is regrowth with use. It is good to start when you are younger so there is not a sharp decline as you age.  PRP works well in younger individuals who want thicker fuller hair with regrowth. PRP is far less intrusvie in your life then the side effects of medications like finasteride.  I recommend seeing a Hair restoration specialist trained in PRP (dermatologist or plastic surgeon) to get a a consultation on their recommendations. 

PRP treatments are fast growing in popularity. While there is a good deal of anecdotal data on its positive effects, there is very little randomized controlled (i.e. high quality) data available. With respect to hair restoration, proponent have demonstrated impacts ranging from slowing/cessation of loss to thickening/strengthening of hair. Some will claim to have demonstrated regrowth.Ultimately, there is very little drawback or risk to the procedure with the potential for significant reward. While the mechanism of action is not entirely clear, the effect will likely be rooted in the native concentration of growth factors in any given patient's plasma. Some will have better quality or more concentrated plasma than others.In my experience, it is a nice option for young people who have yet to reach a stable hair pattern (i.e. not ready for FUE grafting/Neograft) and are looking for an adjunct to traditional medical therapy. That being said, it is important to limit your expectations as not everyone experiences regrowth at the same pace (or at all). However, anecdotally speaking, everyone seems to gain some benefit with regular treatments (annually to bi-annually after 3 treatment induction).As always, discuss your concerns with a board certified plastic surgeon (ABPS), with experience in hair restoration.

PRP results have proven extraordinary and to be effective in both men and woman suffering from hair loss.
Our PRP techniques have shown to accelerate the
regeneration process, and improve overall scalp health. Additionally,
the therapy makes use of the proteins and growth factors that are
included in PRP. These factors, including VEGF and PDGF, have shown to
induce follicular growth and result in the strengthening of miniaturized
hairs making for a fuller, thicker and healthier head of hair.
As
a stand-alone therapy, improvements can be seen as early as 2 months
after treatment. Results are sustained for more than 18 months and are
improved with booster treatments. After your PRP stand-alone treatment,
there is little discomfort and full activity can be resumed.

As a very young man, you will want to maximize your nonsurgical options for maintaining your hair.  PRP is certainly a good option with promising results to date.  There are others, of course. For a real evaluation & solutions, you need an in-person consultation with an experienced dermatologist &/or hair restoration surgeon.