two years old, two year olds

Two years old describes someone or something that has existed for 2 years--
Juinor will be two years old next week.
His phone is two years old; he wants to upgrade.

Two year olds  or two-year-olds is the plural ( more than one ) of  "two year old", meaning a child, a horse, or some other creature that has lived or existed for 2 years--
The Kentucky Derby is for two-year-olds.
This toy is recommended for two-year-olds.

Now that you know that, you can say--
"When the baby is two years old, we will be taking him to a special daycare for two-year-olds."

Note--any number might be substituted for two--
This race is for three-year-olds.
Their little girl is four years old.
Note # 2--"two year old" may describe a person or thing--
They have a two year old daughter.
Thye have a two-year-old child.
He has a two-year-old phone.
He has a two-year-old car.
Note # 3--many people prefer the dashes because the meaning is clearer. A sentence might be describing two things or persons that are a year old each--
"He has two year-old dogs." is awkward.
We would probably write--
"He has two one-year-old dogs." instead.