A 25 year old man had sex with a 14 year old girl and he knew she was 14 but he tells the police that she's 18, will he go to prison?

Probably, if it can be proven.

By the way, the question reads like a hypothetical one. Where did this happen?

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... he tells the police that she's 18 ...

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They can check her birth certificate. Do you perhaps mean he says he thought she was 18? Normally that doesn't matter.

The fact that the girl was 14 and the man is 25, The girl can not consent. Normally his knowledge of girls age, does not matter and is not a defense.

Yes, very likely he will go to jail. There will have to be some proof of the sex

There are other factors; location, relationship, her appearance etc. That will bear on this. The first thing that needs to happen is it needs to be reported to the police which apparently was done. An investigation needs to be made where all relevant facts will be gathered. The police will then present it to the local prosecutor to see what charges need to be filed. The prosecutor will then decide whether to prosecute. Then there may be negotiations for a plea bargain, or the case may be brought before a jury and the jury will decide guilt or innocence.

Too many factors to give a hard and fast answer.

So