Since Catherine seems to favour V neck dresses, both daytime and evening wear, she would need a string with a pendant of some kind to pull it into a V or it would look "off". (And no, she cannot have La Peregrina!)
Why not? It's going up for sale on Dec. 13!
But pearls are so often worn by older women, yet they are said to look their best with young women. At one time, a string of pearls was considered the ideal gift for a girl making her debut into society. And for weddings of course.