The frailty could be as a result of sickness or injury, rather than age though. I know some really frail 20 year olds (sadly, usually from cancer or something equally horrible) and some really robust 70 year olds. Or perhaps frailty is meant as the opposite of muscular - as in a dwarf that is not fat but nonetheless has had a job in which they do very little physical labour. Like a clerk.
I agree it would be good to develop the descriptions so that they only focus on things that deviate significantly from the norm... but I don't think hair/skin should be worked that way. If most dwarves in a fortress are broad of body with brown hair and green eyes I would still want to know this to get an accurate picture. And I can see the potential frustration of never finding out that the average dwarf in your fortress has an extremely long nose. Maybe there could be some sort of page giving an overall fortress description to give us the averages.
The Caves of Iron was founded in the year 10 by the dwarf Urist CleavedMoon. The fortress extends across 12 levels and consists of 20 burrows. The walls of this fortress are mainly microcline and some are engraved. The fortress army consists of 37 dwarves in 8 squads. The Champion of this fortress is Big McLargeHuge, who killed 8 goblins during a siege in the year 87. The main export is olivine crowns. The main import is Dwarven Rum. The dwarves of this fortress are uncommonly rugged and stocky in build. They are taller than normal and tend to be very lean. Their skin is usually olive and their hair is on average copper. They tend to have long, upturned noses. They prefer to keep their beards neatly combed, their sideburns shaven and their moustaches in double braids.
I would then know that Urist McSparsebeard who is described as short, fat, uncommonly frail and with large ears, a braided beard and purple eyes is actually shorter, fatter and frailer than the rest of the fortress and, in addition to his large ears and purple eyes, likely has olive skin and a long nose.
It would also add a nice touch when picturing my fortress - a mass of stocky, tall olive-skinned dwarves with long noses... and the few notable dwarves whose descriptions I've paid attention to standing out from the crowd by virtue of their unique features.
But then again, this is a hell of a lot of work I'd be asking for... maybe something to revise for the next release after this one, though, or a few releases down the line?