Are you in US or foreign country? If you are not a US resident then you don't need to pay for taxes in US, but you'd need to pay for taxes in your country.
He's from the US, so adsense has to tell authorities about him. But for me it was at the very beginning of adsense account that I had to fill in tax information in adsense account. They wouldn't send the first payment before I've sent the tax information, and it was easy to find, right on the homepage of adsense account. I guess if you need to ask this question it's because it's not like for me? Why? Did you get paid one day already by adsense?