How old is the practice of consecration to Mary? I assumed it is a recent invention but Wikipedia says it has origins in the fourth century, then developed into the current practice with de Montfort et al. Is this correct?
Yes, that is correct. People are often surprised at how ancient devotion to Mary actually is, but the documentation is there for anyone who cares to peruse it. When I tell a non-Catholic that we have archeological evidence for liturgical prayers to Mary dating from the 3rd century in Egypt, he is usually incredulous and wants proof. In fact, the find has been published numerous times, with pictures, since it was first made public around the year 1990. But the diggings date back decades earlier than that.
I think based upon the Pope’s request, our bishop has instituted this prayer as well as the St Michael prayer to be said in all the parishes in our diocese at the end of Mass for at least this year. I think that is a good thing. If Mary’s Son is Jesus and we are His Body, then she is also our Mother.