Ira Sargent is enumerated in the 1850 and 1860 US Census under the last name of his step-father, Asa Landon. Ira was born in the 1840s and his father, Clark Sargent, died around 1848. By 1850 his mother had married Asa Landon.
Ira’s 1870 marriage record is probably the first document where he actually provided his name to the records clerk. Chances are someone else gave his name to the 1850 and 1860 census enumerator.
Your relative might have known his “name,” but might never have had a chance to give it to the clerk, enumerator, etc. until after he was “of age.”
Is that why you can’t find your person in any record until they get married?