The report also claims the market is up substantially as a whole, so let's
check the absolute figures: .28 * $13.7B is still larger than .32 * $6.6B,
to the tune of $1.7B, so we shouldn't be too quick to cry for the labels.
The drop-off in proportion (and matching increase of proportion of music
buying amoung the 35+ group) isn't very puzzling either. According to the US
Census Bureau, the Baby Boomers (~30-50 ca. 1998) were nearly a third of
the population in 1995, and at that time the number of 15-29 year old women
had actually declined by 6% from the start of the decade.
>The Baby Boomers (those persons born from 1946 to 1964) continue to concentrate
>population growth within the age groups into which they age. The Baby Boom
>cohort, persons 29 to 47 years old on January 1, 1995, accounted for 79,352,000
>people or 30.3 percent of the total population.
>On the other hand, the number of women 15 to 29 years old on January 1, 1995,
>was 27,059,000, a decrease of 196,000 (-0.7 percent) from 1 year ago, and a
>decrease of 1,668,000 (-5.8 percent) from the 1990 census.
Checking elsewhere on the census site to get estimated figures for 1998, we
see that the 15-24 cohort holding steady at 37 million between 1990 and 1998,
but the 35-50 cohort increasing from 52 to 64 million.
So it looks like the recording industry has managed to increase "manufacturers'
shipments at suggested list prices" from $50/person to $100/person between 1989
and 1998, and underlying population change has taken care of the rest.
 figures from <http://biz.yahoo.com/prnews/990323/dc_riaa_19_1.html>