This is an interesting observation because now that I think about it, most names containing "th" sound pretty elderly or vintage, but
Samantha is one of the only ones that doesn't (the others, in my opinion, being
Bethany,
Annabeth,
Xanthe, and
Athena). I think the "n" in the name softens the "th" sound, too.
Elderly th names (in my opinion):
Bertha,
Ethel,
Martha,
Beth,
Theodosia,
Dorothy,
Agatha,
CynthiaVintage th names (in my opinion):
Elizabeth,
Edith,
Meredith,
Judith,
Thea,
Katherine,
Ruth,
Faith,
Tabitha,
Lilith