Not saying it is not to their advantage to keep younger hens, but I think it also has some advantages for the buyers.
1. No need to set up brooder, alternative housing, etc. for young chicks
2. Chicks can die off unpredictability while getting their first disease exposures, so you won't likely see them all reach adulthood. Not the case with adults
3. Pullets first coming into lay are likely to have oops eggs, like softshells, and possibly more reproductive/egg tract issues in general vs. hens with a full season complete
4. No sexing confusion, you can easily tell grown hens from grown roosters in the majority of breeds
5. They may not lay as much, but still have a good year or two ahead
6. Culling will earn you a grown bird to eat right away, if it comes up