I am retired with a modest retirement and Social Security check. Taxes are taken from my retirement check each month but not from the SS check. At the end of the year I usually end up paying a few hundred dollars to cover the Social Security income.
Four years ago I started a recipe web site just as a hobby. It made little Adsense income until last year when it took off. Like Social Security, Adsense does not deduct taxes during the year. So, I went into shock when I found out I owed $2,400 in tax last April 15.
The point is this...be aware that if you suddenly begin making significant money with Adsense you should get tax advise early in the year. Do not wait until year end to think about any possible tax burden.