This is the 16959th most frequent Russian word.
сорокалетним
"Сорокалетним" means "forty-year-old" in English.
In this sentence, "сорокалетним" is used as an adjective describing "мой отец" with the instrumental case, denoting his age of forty.
Мой отец стал сорокалетним в прошлом году.
My father turned forty years old last year.
Here, "сорокалетним" is an adjective modifying "мужчиной" in the instrumental case, specifying the age of the man they talked to.
Они разговаривали с сорокалетним мужчиной о строительстве.
They talked to a forty-year-old man about construction.
In this example, "сорокалетним" functions to describe the time or age when changes were noticed through the instrumental case.