I have a carte de sejour and want to get one for my 20 (soon to be 21) year old son who wants to spend a couple of years in France working. Does anyone know whether I can get a carte de sejour for a 21 year old child?
The fact that he is your child won't help him visa-wise. As he is over 18 he is considered an adult. A carte de sejour is only a residency permit, but a visa is what you need to enter France for a specific purpose. To live in France for more than 90 days he needs to apply for a visa. To be eligible for a work visa he would have to be sponsored by an employer who is willing and capable, and the visa would have to be applied for from the US.
Alternatively he could apply for a student visa by taking studies at university, perhaps taking a French course, which would allow him to work a restricted amount of hours over the study period, but he would only be allowed to remain in France for the duration of the course.
He needs to qualify for his own visa, as Sarah said. Regroupement familliale is limited to children under the age of 18 and the sponsoring family member needs to have been living legally in France for at least 18 months.