|   Re: 8 years old or 8-year- old? 
 It depends. In a sentence like My son is eight years old
 you have a "normal" number plus plural noun construction.
 but in a sentence like
 I have an 8-year-old son
 8-year-old functions as a single chunk, and is an adjective.  You could replace it with eg
 I have a young son.
 Because it's an adjective it's not pluralised, and because it must be considered as a single chunk, it's hyphenised.
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