Like most things in winemaking, picking grapes at the right time is both an art and a science.
Historically wine grapes have been picked when sugar and acid levels indicate ripeness complimented by the subjective flavour ripeness assessment of the winemaker. There has been considerable research to make this qualitative assessment by the winemaker more quantitative so that we can rely more on the numbers than sensory perception.
There are three basic parameters to ripening grapes: sugar, acid and flavour. Flavour comes into the realm of physiological ripeness and there has been considerable research effort into how to make this parameter more quantifiable. Flavour develops late in the ripening phase in fact it is possible for the sugar and acid to be ripe with the aroma and flavour lagging behind. This is particularly a problem in warmer climates as the grapes can be sugar and acid ripe but not flavour ripe this can cause the need to augment the ferment with acid to rebalance the must.
Red wines have phenolics in the form of tannins and anthocyanins (the colour molecules) that protect the wine as well as the acid although the acid levels are usually lower in reds than whites. Generally red wines that have a lot of tannin when young are good ageing candidates.
We are unable to taste all that may evolve from the grapes as some of the flavour and aroma molecules are bound to glucose molecules and form flavourless glycosides that only release late in ferment as the glucose is converted to alcohol by the yeast and other flavours are only manifested through maturation.
In red grapes the flavour ripeness corelates quite well with colour intensity which is used by some wineries to determine when to pick.
The style of wine produced is heavily influenced by when the grapes are picked and like most things in winemaking this is both an art and a science.