I just had a phone interview and I already wasn’t feeling it because the woman was curt and abrasive but while hanging up she asked me by the way what my age, is this legal?

It caught me so of guard I didn't think to say why she needed that and it was illegal I just answered her. Why is this relevant, I'm not young but I'm not old I'm pretty sure she asked because she didn't want to hire some young thing for the position. I'll guess I'll see if she follows up with an in person interview but I wont be going