differeing age of consent...

[GWMinUS]

And for US Citizens, it does not matter if the Country he might be visiting has a lower age of consent, like 16.
You can be prosecuted if the US Govt finds out and decides to take legal action.
Now, I do not know how the US might look at a 16 yo in Australia deciding to leave home with an older man!
I think the older man would still be in deep shit back in the US. Better the older man adopt the 16 yo!!!

[Armando]

The answer to your question, as far as the UK is concerned, is No.
You cannot be prosecuted unless you have sex with an underage person in the country you are visiting and are prosecuted for it.
If you can believe newspaper articles see here
https://www.theguardian.com/uk/2006/...igrationpolicy

That piece is dated 2006. Another from the same paper indicates almost nothing has been done since then. Seems there already is a reciprocal prosecution 2003 law in England but it is rarely used. Only one prosecution every three years. But is it worth the risk?

https://www.theguardian.com/society/...-sex-offenders