here in vegas, a mature nigra stem grafted onto alba seedling will produce berries earlier than 2 years, but the berries tend to get aborted at the red stage for three or more years.
conversely, quite curious if a nigra seedling stem grafted onto a mature/old(and at its prime) alba or nigra would be more precocious
if the nigra seedlings had alba parentage, that may explain the precocity reported by @pileta
the 'old world' has some of the oldest mulb trees, and being a great continuum of land from portugal all the way to china/korea and long history of agri-trade via silk route , it probably facilitated what @pileta is reporting now in bulgaria! And is main reason why was so curious about @pileta 's whereabouts...
here in north america, we seem to only have rubra as endemic species , and only relatively-recently imported nigras and albas