A limited percentage (maybe 5%) of the wines produced in this world can develop new aromas in the bottle over time and their structures integrate to create smoother wines. Some new aromas (molecules called esters) are created during bottle aging thanks to the reaction between acidity and alcohol. Some other molecules called polyphenols responsible for the colour and tannins present in the wine when bottled decay and give way to compounds responsible for flavours when they decompose, and the result of tannins decaying transform astringent young wines into smooth and pleasurable drinks after laying down. 

The aging process is very complicated and not fully understood yet but it is recognised that the following elements are required if a wine is to age amongst others: - ripe tannins and in decent concentration, good acidity from the grape flesh (in reds and whites), good enough concentration of aromas from good vintage, and use of sulphur dioxide added by the winemaker as an antiseptic and antioxidant. More generally, quality in the viticulture and winemaking is crucial in producing a wine which will age: low yields, and care from the work carried out in the vineyards to bottling will help. Winemaking will also be very important as the ability to extract those components the best from the fruit. Finally the place is also very important and some sites are better than others in producing the best grapes which will then age.