"The oldest professor is eighty years old."
Translation:En yaşlı profesör seksen yaşında.
"yaş + ı + n + da"
'yaş' - age 'ı' - 3rd person poss. suffix n - buffer letter da - locative case, "at"
"At his age," which sound kind of clunky in English, but it is simply the way it is worded in Turkish (and to keep in mind, the possessive suffix does not change according to person. "Ben 23 yaşındayım" for example.