How and why ages the wine?

Ageing is the process of the tannins in the wine reacting with other components until they are unable to stay in solution, where upon they become sediment.

The aromas of the grape are replaced by the bouquet of the aged wine. The colour in the wine either lightens, as in red wines or turns brown, as in white wines.

A hard, tannic red wine will become softer in the mouth, less fruity and more wood/leather in the nose, and generally more complex, as it ages.  

When an oaky white wine ages will become less fruity in the nose, more golden in color, and more complex and subtle in the taste.  

It is impossible to make general rules about how long any given wine will age, but it is certain that many Cabernet Sauvignons will indeed age 5-7 years and more that will age for decades (the richer, more expensive selections).