A job interviewer said: "I am certainly younger than you". Can he said that in an jib interview?

Yes. Nothing wrong with that.

It's just stating a fact isn't it? Would "I have blonder hair than you" also trigger you?

Of course, if he wants to.

People always looking for a lawsuit. If he knows your age & if it's true then of course he can say that to you..

Let it go & move on with your life.

it may not be appropriate or professional, but nothing you can really do about it. though, companies can't discriminate based on age, so if you don't get the job on that situation alone, you may have a case.

ye unless he then said 'So I'm not going to hire you." Kind of odd thing to say though.

Please speak proper English when you are interviewing. It was rude to say, but not much you can do about it. I do not know how either of you looks, for your respective ages, but it could be argued that he/she was simply stating an obvious observation and not discriminating against you for your age. But, you would have a better chance of arguing age discrimination, than someone who was told they were “overqualified”.

If I am to believe you, he can say that because by saying he's younger than you, he's calling you young. You're young; he's younger. That's how the comparative declension in the English language works. Age discrimination is when you're discriminated against for being old, which the Age Discrimination in Employment Act of 1967 (ADEA) defines as 40 or older. But, again, he didn't technically call you old. He called you young, just not as young as he, so legally speaking, even if you are 40 or older, he skirted the line of possible age discrimination, depending on his hiring decision and whether that was a factor, without actually going over it.

possibly. I was asked outright how old I was. (I'm old) That is illegal...however, it's difficult to enforce

If it's true, of course he can say that. It's an odd thing to say but there's nothing illegal about it.

I'm 65 and one of my bosses is 42. I'd think it was strange if he said it, but it's true.