One must be 18 years of age or older to enter this place.

Is the above sentence correct ?
How can one be 18 years of age ?

One can be 18 years old though.

Yes, it's correct: "x years of age" is a set expression meaning the same as "x years old". It can feel a bit wordy and formal; "... must be 18 or over ..." is more conversational.

If I'm, not mistaken, saying one way or the other is correct-they are just variations. It is easy to understand the meaning, therefore, OK to be ignored both speaking and writing. If you look it up on the internet, you'll find both ways taking you to different sites.

 Mr Wordy's reply was promoted to an answer.