Glycation occurs in all living cells. The process of glycation, also known as non-enzymatic glycosylation, is normally regulated by enzymatic activity, which is necessary to regulate the metabolic functioning of molecules.
Glycation occurs when excess glucose molecules stick to collagen and elastin fibers, forming Advanced Glycation End-products (A.G.E.), also known as "glycotoxins." Young collagen and elastin is cross-linked in a particular way that promotes firmness and flexibility. Once these collagen and elastin fibers in the skin are glycated, they lose their normal functions. The body cannot break them down and replace them, so they become stiff and brittle, resulting in wrinkles and sagging, thinning skin.
Too much glycation may affect what type of collagen you can build, which is a huge factor in determining how resistant your skin will be to wrinkling. The damaging effects sugar can have on your looks are clearly evident in diabetics who have a hard time controlling their blood-sugar levels. Diabetics often show the signs of premature aging because they can go for years with undetected high blood sugar, causing them to physically age quicker.
In simple terms, glycation inhibits collagen production, produces free radicals, and increases inflammation, causing skin-sagging and wrinkles.
Further reading - ELLE Magazine - Sugar and Aging: How to Fight Glycation