In your first sentence, it is
"Ace Grit is a 16 year old Vestal boy."
While in the second sentence it is
"Ace Grit is 16 years old."
I can't give you a full reason as to why this is the case, maybe someone else can, however the general rule to follow would be:
If the age (XX) is being used as an adjective before the noun (YY), the correct phrase is "he is an XX year old YY." If the age part of the phrase is coming after the noun then it is "XX years old," as the noun years must agree with the number of years. (Therefore if you are talking about a baby, then they would be "one year old" and the plural would sound wrong.)
This rule works for all ages and number of people, with the exception of a baby, which I pointed out above.
They are sixty years old.
They are sixty year old men.
A few little bits of information not directly linked, but that may be helpful anyway:
You can use the phrase "XX year olds" as a noun if you have lots of people. For example,
The room was full of thirty year olds.
Our target audience is forty year olds.
Also, you may here the phrase "XX years young" which means exactly the same thing, but is often a way of talking about older people without calling them old
My grandad is eighty years young.
This construction cannot really be used as an adjective before the noun, and while the meaning is clear and would potentially be said by a native speaker looking to give a certain effect, you dont commonly hear
He is an eighty year young man.