Can I file an F2B petition for my divorced daughter of over 21 years?
Yes - if they have not re-married and are otherwise eligible. F2B applies to unmarried children over the age of 21. Unmarried in that context does not mean never married. You should note however that there is a very long wait due to limited number of Green Cards available each year in this category. Currently the wait has been at least 7 years, (but 21 years for Mexicans and 11 years for Phillipinos.) See Visa Bulletin at Visa Bulletin For October 2018