Is it possible to somehow change npc age (editing files or anything) without messing with body modding option in Loliphile's version of the game? Because when I change the minimum age on settings to 18 years it sets 99% of npcs in slaver alley to 18 years.
The only thing that comes to my mind is to check your content settings.
I cannot remember what the name of the tab is, but there is one tab that lets you select the abundance of the various age groups (how often a particular age group spawns) from 18 up to about 60ish. It has 3 sections for maculine, feminine, and androgynous characters. So, it is possible that you set one group or all of them to only 18.
Other than that, I have no ideas for you.
As, Draupnir said, it seems to be a known issue and no fix for it right now.