It should either be "She's twenty-nine years old" or "she's a twenty-nine-year-old (woman)." In the first, "twenty-nine years" functions as a adverb phrase modifying the adjective "old". In the second, "twenty-nine-year-old" is a adjective phrase modifying an elliptical noun that was likely mentioned in a prior sentence. https://english.stackexchange.com/questions/84839/what-are-the-parts-of-speech-in-hes-fifty-years-old

The correct form is either: 'She's twenty-nine years old.' or 'She's a twenty-nine year old.' It really doesn't matter whether 'twenty nine' is an adjective, a noun or a cucumber. The important thing about grammar is to get it right.

numeral

She's twenty-nine years old. You write "years" with s, since there are twenty-nine of them. Twenty-nine is a number. "I have a twenty-nine-year-old sister." "My daughter is a twenty-nine-year-old." Twenty-nine-year-old is an adjective.

i think twenty nine is a object, it addition mean full for the following word