Should we pronounce "21st Century" as "twenty first century"? I've heard some pronounced it like "twenty century". Is what they pronoounced okay?
No, "21st Century" is "twenty-first century"!
Twenty first century - native speaker
Students: Are you brave enough to let our tutors analyse your pronunciation?
any native english speaker's opinion?
 abbie1948's reply was promoted to an answer.
Students: We have free audio pronunciation exercises.
It's possible that you heard this spoken in very rapid speech, Jobb. In such cases, the sound of 'first' can be reduced and you might have missed it.

On most things, especially related to speech, I'd say that you can put your trust in Pieanne's judgement.