I was looking for "of eight years", and I saw "octoni, ae, a." Then I saw: Septuennis "of seven years, seven years old. (Puer Septuennis, a seven years old boy) Quinquennis: "of five years, five years old (Puer quinquennis, a five years old boy) Could I say "puer octonnis", or should I use the genitive "Puer octonorum annorum"?
Just to supplement Sebastian's answer, octoni is the distributive adjective for octo and it means "eight times" or "eight at a time." It is not the same type of word as septuennis. For that, Sebastian is correct, and you want octennis.
However, you can use octonus for age, and it can be used as a mere substitute for octo. Just take Ovid's Metamorphoses (5.50) as an example:
bis adhuc octonis integer annis
Now untouched at sixteen (twice eight-times) years old
And it's not limited to poetry, as Pliny the Elder often uses it in his Historia Naturalis as well (e.g. 8.201, 10.107).
columbae et turtures octonis annis vivunt.
Pigeons and turtle-doves live for eight years. (Pliny, NH 10.107)
That said, you'll still need a noun to go with it.
The word is octennis. You can say puer octennis – but it's an extremely unusual word and not classical.
More common ways to express the idea are:
- puer octo annos natus – an eight year old boy
- puer octo annorum – a boy of eight years
- puer nonum annum agens – a boy living in his ninth year
Thanks, Sebastian. I don't care about the word being classical. Actually, I don't even like classical Latin. Feb 9 at 16:54