I would say, first of all, that I’m not at all an advocate of baptismal regeneration. I don’t think that baptism conveys regeneration either for infants or for adults. Regeneration is a sovereign work of God the Holy Spirit.
Now, baptism is a sign of regeneration, but that doesn’t mean the sign is conveyed automatically by the sacrament itself. As a Protestant, I believe that the sacraments are effective by and through faith.
In the operation of baptism, God makes a promise to His people and to their seed. If they come to faith (and that would presume they were regenerate because they couldn’t be coming to faith unless they were regenerate), then they, along with all who believe, would enjoy all of the benefits that were wrought for us by Jesus Christ. We participate in His death and resurrection, are cleansed of our sin, and all the rest.
Baptism is a sign of all these benefits, but it doesn’t automatically convey salvation any more than circumcision automatically conveyed salvation in the Old Testament, as Paul labors in Romans (Rom. 2:28–29).
So, we can’t assume that just because somebody has been baptized that, therefore, they are regenerate.