I have a beginner Jupiter 511 flute that I think is about 10 years old and I know it is over 5 years old, but I don't know how old it is. There are some things that do not work on it, like the low C. I have taken it to a few repair people with no avail. Is it worth the effort or should I just wait a few years and buy a semi-pro flute?

You obviously aren't taking it to a reputable repair technician if they can't fix it. Try taking it to someone that repairs only flutes and get references if need be. If it still can't be fixed, then yes, look for another flute. Do they tell you what is wrong with it and why they can't fix it?

I am not sure what you would do.....I know that the older Jupiters were not very good so the c might not ever have worked....