From the second link: Some Indian Ringneck males will ring out prior to age 2...
A Look At Indian Ring-Necked Parakeets “The most noticeable visual difference between male and female is that the male gets a ring and the female does not,” said Bush. “The ring does not appear until after two years, so until then, know there is real visible difference.” According to Baum, “males are visual between the ages of 18 months and 3 years, when they get their very distinct rings, the color of which varies depending on the color of the body feathers.” She noted that females have “a very faint, thin ring around their neck, as do all juveniles.”
Most of us have had our birds DNA tested if we know their sexes as most birds you cannot tell any other way (macaws, amazons, conures there are no visual differences in the sexes)