I have no knowledge about this within the parameters you describe, but I would imagine that if such material was made long ago then anything containing what we currently consider underage performers would hopefully have been purged from their libraries a long time ago, never to see the light of day. I would also hope that the same goes for any digital copies floating around in the ether.
As to the question of whether such material would be "safe" to watch simply because it is of a certain vintage then you should consider this: There are countries that do not legislate against what we regard as underage performers in any way, shape of form. Would it therefore be ok to watch such material simply because it was produced beyond the reach of the current law wherever you are?
Simplest way to deal with this question is "If in doubt, don't."