I know a
Marguerite who is about 30. I think it's totally usable. I like it a lot better than
Margot.
Margot sounds so harsh to me.
Marguerite is softer and like sunflowers! And, like someone else mentioned, you could really use
Margot as a nickname for
Marguerite if you wanted. Though personally I think
Rita would make a good nickname for
MArguerite.