The general consensus amongst dermatologists is to start using retinol products for anti-aging reasons in your 20s. From evening out skin tone and texture to boosting radiance and preventing blemishes, it is a real multitasker that can suit all skin types.

Everyone’s complexion ages differently — some may need it earlier, others later — if you are dealing with breakouts, pigmentation from UV rays or age spots, fine lines from collagen breakdown or dullness, you will benefit a great deal from adding retinol to your regimen.

How do retinoids work?
Inside our skin, we have retinol receptors called retinoic acid receptors (RAR) and retinoid X receptors (RXR). When you apply prescription retinol, known as retin-A or retinoic acid, the retinol acts on those receptors to speed up skin cell production. Old cells die faster, making way for new, young, healthy cells. Retinol also encourages collagen production so, essentially, it triggers your skin to act younger.
Over-the-counter retinol has to go through a conversion process once inside the skin before it can act on the receptors, which means it is less likely to irritate than stronger prescription retinol.

A. Retinoid Acid
Retinoic Acid (also known as retin-A or tretinoin) is seriously potent and only available with a prescription.

Conversion: No conversion needed. Already in the active form. It binds directly to the retinoid receptors to activate a biological response. 
Efficacy: Highly effective as it binds directly to the retinoid receptors and activates a biological response. It acts immediately on receptors for fast and furious results.
Side Effects: Depending on the percentage it can cause severe and very uncomfortable side effects, including redness and peeling - even if you don't have sensitive skin.

B. Hydroxypinacolone Retinoate
Also known as granactive retinoid. It's an ester of the prescription retinoic acid and the hottest new retinol on the block. This molecule is unique in that it processes innate retinoic acid activity, and thus does not need to undergo metabolic breakdown to achieve skin benefits. It has been demonstrated to be more stable and cause less skin irritation than Retinoic acid, or any other less effective form of Vitamin A. 

Conversion: It works directly on receptors in the skin just like retinoid acid, but has little-to-no downtime making it an ideal choice for anyone who does not want to risk irritation and wants quick results.
Efficacy: Highly effective as it binds directly to the retinoid receptors and activates a biological response. It acts immediately on receptors for fast and furious results.
Side Effects: No side effects, as it does not bind to the specific receptor that causes the side effects. 

C. Retinol
An OTC vitamin A derivate that we have all heard of.

Conversion: Once applied, it sinks into the skin where it is converted to retinaldehyde and then to retinoic acid.
Efficacy: Potency and efficacy depend on product formulation, pH, and Retinol concentration.
Side Effects: Depending on the percentage it can cause some redness and peeling if your skin is slightly sensitive. The best way to avoid side effects is to use it two nights a week, see how your skin reacts and once it becomes more accustomed to the retinol ingredient, gradually work your way up to three nights and then four.