An 18yo provider is almost certainly managed. It may say "indie", but rest assured, she's turning over at least half of the fee to some dude at the end of the day.
Since almost all AMP girls are imports (literally by definition), circumstances would indicate that the earliest a girl could come over would be 22-23yo. The youngest AMP girls I know out west, one is 24 and has been working 2 years now, the other 26 with same experience.
You want young, you need to go to the source...PI is your best bet, followed by China and Thailand.