An underextracted coffee has the fast dissolving compounds that, if the coffee is good, you want.
This includes all the fruit acids and the short chain sugars. It will also have the medium speed compounds (unless you really screwed up) like the toasty, rooty, herbal, and nutty flavors. The mark of underextraction is that you are missing the caramels, along with the cola and cocoa flavors, and the long chain compounds that give the sensation of body and that reduce astringency (i.e. take the edge off the taste).
A fully extracted coffee is just as acidic as an underextracted one, it just has the sweetness and body to balance the sour tastes. When the coffee is very ripe and has a lot of sugar, a relatively underextracted cup will have more clarity and still be sweet; but that is rare. Older coffee looses its less stable, short chain compounds more quickly than the long chain ones; so coffee should be extracted less as it ages.
This is why you lengthen the usable shot making window by starting with fine grinds and low doses on fresh coffee, and grinding coarser and dosing higher as the coffee stales.
한줄 요약 : 오래된 커피는 굵게 많이 - 신선한 커피는 가늘고 적게