I-864 part 3 has

the following instruction:

Accompanying Family Members You are Sponsoring.The immigrant you are sponsoring may be bringing a spouse and/or children to the United States. If the spouse and/orchildren will be traveling with the immigrant, or within sixmonths of the immigrant's entry to the United States and youare sponsoring them, you should list the names and other requested information on the lines provided. If any dependents are not immigrating, will be immigrating more than 6 months after the sponsored alien arrives in the United States, or you are not sponsoring them, do not list their nameshere. A separate Form I-864 will be required for them when they apply for their immigrant visas.

I am a US citizen and sponsored my sister to immigrate to USA. It is Family Fourth Preference (F4): Brothers and sisters of United States citizens, and their spouses and children.

She will get the visa next year. Is there an age limit for her daughter (she is not married)? If there is one, how to calculate it?

Thank you very much.