Is the word order in the quantity correct in the following sentence?
The boy is 3 years and a half old.
If not what would be the right way to say it?
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 [dollars] and [cents]
No, you should say "The boy is three and a half years old."