18-feet tall OR 18-foot tall

[Freeguy]

Which one is correct and why?

1. That sculpture, called the Earth Goddess, is 18-feet tall!
2. That sculpture, called the Earth Goddess, is 18-foot tall!

 

[Raymott]

Which one is correct and why?

1. That sculpture, called the Earth Goddess, is 18 feet tall!
2. The Earth Goddess, is an 18-foot tall sculpture!

Note the two differences. In the first sentence "18 feet" is a noun. In the second "18-foot" is an adjective.
There are possibly regional differences.

 

[Freeguy]

Why "Number-foot" serves as an adjective?

 

[Raymott]

Why "Number-foot" serves as an adjective?

Because it qualifies sculpture. It's an 18-foot sculpture. I'd say that "18-foot tall" is an adjectival phrase.
However, the sculpture measures 18 feet. That's a noun, perhaps acting as an adjective in sentence one.
However you parse them, the phrasing is correct.
Why, how would you label them?

 

[Freeguy]

It's hard to label them. ;-)

 

[Matthew Wai]

Why "Number-foot" serves as an adjective?

Does it serve as an adjective as "number-year-old" does?

 

[Rover_KE]

Yes, it does.