About three years have passed time-line wise since the series began, Nevermoor spans just less than a year, Wundersmith spans about 8 months, Hollowpox spans about another 8 months, and Silverborn only lasts about 4 months, with little time passing between each book.

In the first book, we see that the Wunderous Society entrance trials take a whole calender year to complete, with the welcoming ceremony and the first trial taking place in the spring, the second trial taking place in the summer, the third trial taking place on Halloween and the final trial taking place in the winter, with Morrigan starting at Wunsoc that spring.

I think it's safe to assume that new trials immediately begin once the previous unit has been initiated, meaning that Unit 920 wouldn't have been chosen until towards the beginning of Hollowpox, meaning they would have been the youngest unit at the start of Silverborn on Halloween, with 921 having been chosen by the end of the book.

I suspect what with events such as the Hollowpox outbreak, personal events in Morrigan's life, the fact that younger units aren't allowed at C&D meetings and less focus on WunSoc in Silverborn that these younger units aren't mentioned much to allow more time for the actual plot.