Debt collectors can report the date they received their collection authority, either by way of assignment from a creditor or their purchase of the debt, as the date opened of their collection.
As for how the collection is scored by the FICO algorithm, there is much anecdotal experience posted on the site to indicate that the date opened governs the scoring.
Since FairIsaac has not, to my knowledge, published the specifics of the date used and aging effect of collections, that appears to be the best info available to us.
However, I personally dont see the logic in such scoring, as just as a collection is excluded based on the DOFD of the OC account, it would make sense to me that its scoring should not be based on the arbitray date that collection authority was ultimately assigned, but rather on the date that began the delinquency on the OC account.
Another of those mysteries of a secret scoring algorithm.