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.