is there anyone can help me in this question?

which sentence is right?
1. I have a two years old cat
2. I have a two year old cat
...
'two' is plural and we often say :
my cat is two years old. Its correct, no doubt.
but if i want to express this sentence in another way like
i have a two years old cat
is it still right?

can u help me with some similar examples to make it clear

thanks in advance