When should my daughter start singing lessons?

Any age is good! Go for it.

6 is a perfect age to start. Your daughter will be far ahead of the curve.

6 is a great age to start

Start as soon as possible but make sure its what she wants to do too.

Now is as good a time as any!

If money isn't an issue, then I see no harm starting at this age if it's what she wants! When I was in primary school, the age they started offering extra curricular musical instrument/singing tuition to children was the year when we were 6/7 - I had just turned 7 when I started playing the flute and I played it for years and enjoyed it very much! Looking back, I'd have preferred singing lessons though, I have a shocker of a voice now lol!!

Do you belong to a religious organization that has a childrens' choir? That might be the best way to start. My older daughter is also a natural born singer and she was in various choruses and choirs from elementary school until junior high. She was in a childrens' choir that was run by the music program at a local university and was in many musicals. But somehow she got it into her head that she was not as good as people said she was and gave it up. If you heard her sing, you would wonder why she lost the desire. I am still trying to figure it out. The biggest question you have to ask is can you face the possibility of rejection? There might be people who don't think she has the talent that you think she has. And how is she with rejection? You have nothing to lose by exploring local childrens' groups, which might cost little or next to nothing.