No persons in the cockpit (correct me if i am wrong) but after sept 11th only applies to usa and is only a sop in european airlines?
This predates 9/11 in the US, I remember asking when on a United flight in 1998 if I could visit up-front. The Virgin crew a few months earlier were most obliging, but they're not a US airline. What did change after 9/11 was the fact that the cockpit door was made into more of a barrier to discourage those who weren't polite enough to ask.