Note that "The Feds" or any other law enforcement agency would not wreck their case by saying at any point they are 18 or older. Instead, they typically play as under-age models looking for a break into the business - clearly stating they are under 18, and showing the vulnerability of needing an adult to help them.
Playing on-line with someone who states legal age for where they are is not prosecutable - they can not show knowledge of a crime; it's as legal as buying something on e-bay that turns out to be stolen goods.
Kudos to those who abruptly broke things off as soon as they had knowledge the other person was underage - it was the right thing to do. If the other person needed therapy "due to that abandonment", the therapy would have been needed anyway; they were getting into things they couldn't truly fathom.

- bracemaiden