What made you jump to that conclusion? As depicted in Where No One Has Gone Before, I took Tasha to be at least 18. Not that I'm saying that makes it OK - it is still terrible! But it is quite a leap from "rape gangs" to "child rape gangs", and I can't recall that being said anywhere.
From "The Naked Now".
TASHA: Do you know how old I was when I was abandoned?
DATA: Chronological age? No, I am afraid I am not familiar with
TASHA: Five. Five years old, but I survived. I learned how to stay alive, how to avoid the rape gangs. I was fifteen before I escaped.
I think her flashback to that scene in "Where No One Has Gone Before" was simply putting her current age into that situation, not representing her true age at that moment. So taken to it's logical conclusion, Tasha had to avoid the rape gangs from the ages of five to fifteen, not eighteen.