This is the 7278th most frequent Russian word.
восьмилетний
Eight-year-old.
Here, 'восьмилетний' is used as an adjective to describe the duration of the journey, emphasizing that it lasted for eight years.
Это было восьмилетнее путешествие.
It was an eight-year-long journey.
In this sentence, 'восьмилетний' describes the age of the son, implying he is eight years old.
У него есть восьмилетний сын.
He has an eight-year-old son.
The word 'восьмилетняя' refers to the age of the dog, indicating it has lived for eight years.