In the tool for 'item modifications by age' there is an items.dateaccessioned option and a note that this will be used if left empty:

<span class="hint">If not set then items.dateaccessioned will be used</span>

It would be a little easier to make the field required, keeping the empty option, but enforcing people to pick someting.

Alternatively the empty option could be removed with items.dateaccessioned the first option, but I believe the first option would be a little cleaner.

Eventually older entries in the db will need updating as well.