Probably they are implying the mentally retarded who grow up to physical maturity without ever achieving the use of reason.
Probably, infants and those LIKE infants (aka those who are mentally retarded). I recall, however, that St. Benedict Center made more of this passage than that.
At one point St. Ambrose, in his (in)famous oration about Valentinian, posits that Valentinian may have been "washed" by his "piety and desire". Elsewhere, however, St. Ambrose states that not even a good catechumen can be saved if he dies before receiving the Sacrament. So this has puzzled many readers, written off as a contradiction or change of opinion.
But elsewhere in the Valentinian oration, he states that not even the martyrs are crowned even if they are "washed". So St. Ambrose is distinguishing between "washing" and "crowning" ... and these two reflect the two different effects of the Sacrament of Baptism -- 1) remission of sin and 2) reception of the Baptismal character. So St. Ambrose holds that unbaptized martyrs (he did seem to believe there were some, while Father Feeney does not think so) receive a washing, a remission of sin, but not a "crowning". That "crowning" (referred to by other Fathers as glory, and as making one fit for the "kingdom"). So this too is consistent with a distinction between justification (remission of sin) and salvation (ability to enter the beatific vision, i.e. enter the Kingdom). Pope St. Siricius dogmatically taught that NO ONE can enter the Kingdom without the Sacrament even if they desired to receive it.
There were theologians at the time of Trent who made the same distinction, holding, for instance, that infidels could be justified but not saved. So the missing piece is, what happens in the next life to those who have been justified but not saved?