Word order in fractional quantities
Number quantities in English should be written with the number before the thing being counted. That is,
There are [quantity][type of quantity]
For example, the following are correct:
He is [three][years old]
He is [three and a half][years old]
This also works with other quantities, like weight or mass:
I have [5 and a quarter][pounds of apples]
I have [5][dollars] and [42][cents]
No, you should say "The boy is three and a half years old."