In England this is definitely illegal.
https://www.legislation.gov.uk/ukpga/2003/42/section/47
https://www.legislation.gov.uk/ukpga/2003/42/section/53A
The first link describes some form of sexual offense, but is only strict liability if the person is under 13. Quoting the link:
"A person(A) commits an offense if..."
"(c)either—
(i)B is under 18, and A does not reasonably believe that B is 18 or over, or
(ii)B is under 13."
The second link is an anti-prostitution offense, which does not match the theoretical example or Minsky's case, and is punishable by fine.
The first link describes some form of sexual offense, but is only strict liability if the person is under 13. Quoting the link:
"A person(A) commits an offense if..."
"(c)either—
(i)B is under 18, and A does not reasonably believe that B is 18 or over, or
(ii)B is under 13."
The second link is an anti-prostitution offense, which does not match the theoretical example or Minsky's case, and is punishable by fine.