It is OK in the USA. I've done it a number of times. It is not approved for international flights because of the ICAO standard. ICAO actually has stated that iit is not required but maintains it because some authorities do insist on the requirement. The present law in the USA says that if ICAO drops the one pilot under 60 rule for international flights there is no longer any restriction for US crews on international flights.
Thank you! Now can you give us a link to verify this, and who are "some authorities" in this concept?
Just curious.