On the left extreme is very young and on the right extreme is very old, but what would you guys wager are actual ages for each bump of the slider? My take is the left extreme is twenty and the right extreme is somewhere in the eighties.

Here are the complexion names for a male Nord if that helps:

SkinHeadMaleNord
SkinHeadMaleNordComplexion_Age40
SkinHeadMaleNordComplexion_Age40Rough
SkinHeadMaleNordComplexion_Age50
SkinHeadMaleNordComplexion_Rough01
SkinHeadMaleNordComplexion_Rough02

So the youngest is forty or younger?

I've always kind of assumed the first complexion was about age 20 or so. But I don't know how accurate that is.

It would make sense to start from twenty and move up ten years for every bump of the slider.

Well, no one ages at the same rate, especially people in different classes. The lowest slider may be 20 for a commoner or field worker but it's probably what a 40-year-old nobleman might look like.

Personally, I think it's 15 like it supposedly was back in Oblivion, but I always use it for characters ages 20-30, since none of my characters are younger than 24.

I'm just gauging age is easier than weight.