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SCENARIO 1 Would it be correct to say (option 1)"Johnson is a 0 years old newborn boy" or (option 2)"Johnson is a 0 year old newborn boy"?
SCENARIO 2 What about this: would it be (option x)"Johnson is 0 years old", (option y)"Johnson is 0 year old" or (option z)"Johnson is a 0-year-old"?
I have done some research and I could not find anything specifically about 0-year-old, because I assume it is different than the rest of the numbers.
In both scenarios, the second option is the technically correct choice: Johnson is 0 years old, or Johnson is a 0-year-old newborn boy.
I say technically correct because, though these phrases are grammatically correct, they are unnatural: both '0-year-old' and 'is zero years old' are unusual in almost all contexts. Children younger than one year of age almost always have their ages counted in months, weeks, days, or even hours. It is common to hear mothers talk about their 'three-month old child,' or say that their baby 'is just two days old.'
So long as there is more than one of whatever unit of time is in question, more complex or mixed units are also used: '14 weeks,' 'three and a half months,' '18 months,' and 'a year and a half,' are all commonly used to describe the age of a child. Thus, 'half a year' and 'half-year-old' are much, much less common than 'six months' and 'six-month-old.'
Also worth noting is that the word 'newborn' is usually reserved for a very young baby, often less than a few weeks old. If you told me your child was a newborn, it would already suggest they are far less than a full year old.
If "0-year-old" can't be found it must be because we don't say it.
SCENARIO 1 We say "Johnson is a newborn baby [boy]". "boy" isn't needed if his name tells us his sex. (I've never heard Johnson as a given name, but it sounds like a boy.)
SCENARIO 2 If we need to be precise we say, "Johnson is a [6]-month-old baby". Otherwise "Johnson is not yet a year old" or "Johnson is only a few months old."
There's two issues here. One is the use of "year old" vs "years old" vs "x-year-old". The other, which appears to be the question you intended, is whether the rules change when using the number 0 rather than higher numbers.
When we use multiple words together an an adjective, we hyphenate them, so only "x-year-old" or "x-years-old" are possible. Further, when using a number in a multi-word adjective phrase like this, the noun is always singular, no matter what the number is: "a one-year-old child" and "a ten-year-old child" are both correct. This means that only "a zero-year-old child" is possible" in your sentence. Same goes for "Johnson is a zero-year-old."
Things do change with the number zero. While it's grammatically correct to say "zero-year-old", we almost never say it. If Johnson is a newborn, then it's enough to say he's a newborn. Stating his age is redundant. Further, at ages younger than one year, we only refer to children's ages in units smaller than years, so all of these are correct:
Johnson is a newborn.
Johnson is a one-hour-old boy.
Johnson is a two-week-old.
Johnson is two months old. (no "a")
Johnson is an eleven-month-old.
Johnson is an eleven-month-old boy.
