Rare, formal term for an adult, literally 'person of full age'. Not used in everyday conversation; modern Japanese prefers 大人 (おとな) or 成人 (せいじん).
The word 丁年者 can be found in old legal documents and the like.
Compound of 丁年 (ていねん, 'full age, adulthood') + 者 (しゃ, 'person'). The exact historical derivation is uncertain; the word is rare in modern usage.