Filing I-485 immediately afterI-140 approval

[mcytpark]

I am currently residing in US and ready to finish my RN studies soon. I have few potential employers who are ready to hire me and sponsor my filing applications for I-140 and then I-485. I have few questions to clarify about filing applications.
1. Am I allowed to file I-485 immediately after the approval of I-140? One source of information is telling me I have to wait until my priority date becomes current (Est 6 years)
2. If I do a premium process for I-140, how long does it take for the approval?
3. When I file for I-485, what is the minimum dependant age that I can include with the application? Is it 21 or 26 years old? My concern is that my daughter becomes ineligible under my I-485 application.
I appreciate your answers.

[Jackolantern]

Your priority date must be current when your I-485 is filed.

If the priority date becomes current while the I-140 is still pending, the I-485 may be filed without waiting for I-140 approval.

Dependent children must be younger than 21 to be eligible for a derivative green card. However, the CSPA has a provision that allows some over-21 children to qualify if they were under 21 when the I-140 was filed.