This is a question I have been wondering about myself, I flew recently, couple of months ago, on a UK domestic flight, not a G reg Aircraft.
I was seated in row 3 so had a good view of the flight deck.
A boy of around 12 or 13 years of age was allowed into the cockpit whilst we were on the ground and I thought to myself remember the days when I did the same thing, I was chuffed that the flight deck were so good with him, they even allowed him to make a PA.
The boy returned to his seat however to my surprise he was summoned to the flight deck again once we had reached TOC where he remained for the duration of the flight, he again was allowed to make a PA giving the flight progress.
Don't get me wrong, I thought it was a really nice gesture, I remember the dream coming true for me many moons ago, but are the rules different for non UK reg operators even when operating within the UK.