"Their" is grammatically plural, even though what it refers to may be singular, plural, or neither (or, in some cases, may be unknown). "You" is also grammatica

"Their" is grammatically plural, even though what it refers to may be singular, plural, or neither (or, in some cases, may be unknown). "You" is also grammatically plural, even though what it refers to may be singular, plural, or neither. (Singular they seems to be from the 14th century, so it isn't really new.)