Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence.

Can radiometric or isometric dating methods be used to determine the age of sedimentary rocks?

The age of new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating. Thus, different crystal grains can yield different ages. With sedimentary rocks, one would end up dating the individual grains of sediment comprising the rock, not the rock as a whole.