OLP is a type of synesthesia in which ordinal sequences (numbers, days, etc) have personalities, emotions or gender.
For example, the number 3 may be cheeky, the number 1 may be bossy.
Research in this area of synesthesia is advancing rapidly as we look at the childhood origins of OLP.
As far back as 1893, Théodore Flournoy wrote about synesthetic personification. He gave several case examples as well.
More recently in 2007 Julia Simner and Emma Holenstein wrote a paper entitled Ordinal Linguistic Personification as a Variant of Synesthesia where they argued that OLP should be considered a form of synesthesia, sharing the same neurodevelopmental basis.
In 2018 Eiko Matsuda, Yoshihiro S. Okazaki, Michiko Asano, and Kazuhiko Yokosawa published a study examining how Children personify non-living objects.
They hypothesised that children would attribute personalities to numbers, more often than adults. They found this to be true but also that “children tended to assign unique and exclusive descriptions to each number from zero to nine”.
They argue that this diminishes over time and that the personification of numbers actually works as a clue for children so they can comprehend the relationships between numbers better.
In short, OLP seems to be another form of synesthesia that is reasonably widespread in childhood, serving a useful function for learning and discrimination but which is outgrown by most but not all of us.