Yes, "two-year-olds" is hyphenated when used as a compound adjective preceding a noun, such as in "two-year-olds play together." The hyphens help clarify that "two" and "year" modify "olds" collectively. However, when referring to the age alone, you would simply say "two years old."
Hyphenated
two year olds can sleep in both
not two year olds they have to be like 7
The plural form of two-year-old is two-year-olds.
yes
Two-year-olds have accidents.
No, "year long" is not hyphenated when used as an adverbial phrase, such as "The project will last year long." However, when used as a compound adjective before a noun, it should be hyphenated, as in "a year-long project."
It should be hyphenated.
To do what?
You seem to be having trouble figuring out which word is the noun that needs to be the plural. Which thing do you have more of - twenty, year, or old? Twenty years is the modifier for old. You have two twenty year olds.
Pre construction - two words not hyphenated.
Thoroughbreds have to be a minimum of two years old to race. All races have their own age requirements. There are races for two-year-olds, three-year-olds (which are the more widely known ones like the Kentucky Derby, Preakness, and Belmont), three-year-olds and up, and four-year-olds and up.