Sorry, I didn't mean to gloss over your question, I assumed it was inferred that 1 1/2 is plural since it's, by definition, more than one.
A very common misstep people take at least here in the States is to say "1 1/2 year old" which is wrong, but it's used a lot anyways, kind of like the word "irregardless" which is not considered a proper English word and has been much maligned over time, but it's a very real word to tens of millions of people.
So in answer to your first question, no hyphen. In answer to your second question, well that's tricky because you cited both forms and both are correct under the right circumstances. Let me summarize.
"My kid is one and a half years old"
"My kid is a year and a half old"
Does this make sense? Technically you can use either construct, but with the number you gave me, which is 1 1/2, the correct form is to use it as a plural.
When you're writing out sentences with numerals, try to avoid acutally using numerals. Use the equivalent words for then insteas. Writing numerals in a sentence is considered a shortcut but the downside is that it can present some odd paradoxes that don't always make sense.
-Nah-