Wines that will mature are those that are balanced. For white wines, balance is achieved amongst the three aspects – alcohol, sweetness and acidity. For red wines, the astringency of tannins enters the equation.
In assessing red wines conceptually, the youthful red wine should be balanced in terms of sweetness, acidity, alcohol and tannin. That is, you should be able to taste all those components and in tasting them, no single component stands out above the others in intensity. With reference to the diagram, (modified from Sentosphere that produces a tasting kit of sensations and an aroma kit), the pink quadrilateral should have sides that are equal. The larger the quadrilateral, the longer the wine will last in your cellar.
That is, you will taste lots of tannin but it is matched by lots of the other components. However, there should be enough fruit flavours and aromas to accompany.
Youthful wines that are in disequilibrium or are unbalanced will never ‘come round’. Therefore the comment that you sometimes overhear afficionados that ‘oh, the wine is unbalanced but give it time and it will come around’ is a misnomer. If a wine is undrinkable when it is youthful, it will never drink well when it is aged.