Let's take a look at what this method does:
What you measure is the thickness of the subcutaneous fat layer on your suprailliac. That does not give you your total body fat percentage directly because body fat distribution varies from person to person.
So what you need is an estimate of your total body fat percentage in relation to the fat on your suprailliac. To do this statistical models are used. So a group of people has these measures taken and their body fat percentage measured (water weighting, x-ray, etc) and the most likely formula of the relation between the measured quantity and body fat is searched (simplified description).
These models which relate skinfold thickness to body fat percentage often include more variables than just one skinfold. Most are sex specific, some take up to 7 skinfolds, bmi, hight, waist circumference, and some, like the Accumeasure chart, use age as an influence on body fat distribution.
Note that all of these estimates are (meaning should be) based on statistics thus these estimates never will fit all. So how good they are depends on how specific the group was the estimates are based on and how well you fit into it.
So to sum up: The idea is to relate an easy measurable quantity (skinfold thickness) to a hidden one (body fat percentage). This relation is most likely unique to each person depending on genetics and lifestyle. So the best that can be done is to gather enough samples to allow a statical estimation of the relation which will give an error depending on the statistics you gathered and the person that is measured.
And now to the wikipedia:
Some models partially address this problem by including age as a
variable in the statistics and the resulting formula. Older
individuals are found to have a lower body density for the same
skinfold measurements, which is assumed to signify a higher body fat
percentage. However, older, highly athletic individuals might not fit
this assumption, causing the formulas to underestimate their body
Meaning: Old people are found to have a higher body fat percentage (statistically) for the same skinfold thickness, but that's not true for all old people (hence statistically).
Regarding the shifting ideals: I can't find it anywhere right now but I read some where that with age a higher body fat percentage is found to be healthier. I think it was on a discussion on bmi where they found that a the bmi limits don't hold for old people and that the ideal bmi for them should be higher (independent of the point that bmi is a crappy measurement).