I'd not heard of yeast scarring before, so I researched a bit.
From Replicative ageing and senescence in Saccharomyces cerevisiae and the impact on brewing fermentations
Ageing is the predetermined progressive transition of an individual
cell from youth to old age that finally culminates in death. Yeast
replicative ageing is a function of the number of divisions undertaken
by an individual cell, and may be measured by enumerating the number
of bud scars on the cell surface.
So, scarring is used as a direct measure of the yeast's age. But it's not thought that the scars themselves cause deterioration of the yeast:
[...] increasing the deposition of chitin, the major component of bud scars [...] has little effect on longevity.
but rather other factors, such as genetics, play a key role in the vitality of the yeast:
It has been postulated that ageing occurs either by a genetic inhibition of metabolically
essential proteins and enzymes, such as DNA repair enzymes or antioxidants
An aged yeast cell exhibits a
distinct morphology and physiology from that of younger cells.
It's known that acid washing kills off weaker yeast cells and bacteria, leaving the stronger yeast behind. Although I can't find a direct reference, presumably the morphological and physiological changes make the older cells weaker, and so they are killed off during the acid wash, leaving only young/middle generation cells behind.
If you have a conical fermentor, another method is to crop and repitch yeast from the top of the cone. This article - Yeast Storage and Fermentation: Effects on Viability and Flavor Production offers a lot of information about yeast handling and touches on yeast scarring and acid washing.
Benefits Claimed for Acid Washing
De-flocculation and ease of handling
Reduced lag period in fermentation
Cleaning the cell surface
Reduced time to achieve desired diacetyl
It doesn't specifically mention removal of old cells, however, the article does include a graph showing the number of scars observed from yeast cropped from a range of 12 regions from the bottom to the top of the cone. Those nearest the top were newest and consequently had the fewest scars.
Both articles mention that the bottom of the cone is usually dumped, and only yeast from the top of the cone is repitched. They both mention that pitching younger cells reduces the off-flavors associated with stressed yeast.
To sum up - you want to pitch only young/middle generation cells from the slurry to avoid off-flavors, and these can be obtained either by acid washing or by cropping from the top of the cone.