Depending on how we structure the sentence we can say He is a five year old boy or He is five years old We don't say he is a five years old boy or he is five year old boy

In the expression "5 year old," the word ''year" functions as an adjective and we do not use the plural form of the word. ( He is a five year old boy.) In the other expression, "5 years old," the word "year" is a noun and we thus use the plural form. (He is five years old.)

I'm unsure if '.5 years old' was intentional or not. If that is what you are asking, then .5 would be a simpler way of saying 'point five', which is again a simplification of saying 'zero point five'. That means that someone who is .5 years old is actually 0.5 (half a year) old. If the question was about the use of 'year' vs. 'years', then the above comments are correct.