When I am using Google translate on "I am almost 18", I get:
Ich bin fast 18.
Ich bin beinahe 18.
Are this sentences correct? What is most used? Are there any other popular forms?
Yes, those sentences are correct. Most used is the first sentence
The other sentence sounds somewhat awkward. Beinahe has - imho - a connotation that means that there had almost something happened.
So, I'd recommend not to use beinahe.
A typical alternative is
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Yes, both are correct. A somewhat more verbose, but also correct variant, would be to say