The IRS gets a record of your balances, as Christian points out, and a 1099-R which shows the distributions. They know your age. It's an easy calculation. However, in my experience, they don't seem to do the math, and instead, seem to rely upon "self reporting."   That said, I sure wouldn't want to mess up in this department, because the penalty (excise tax) is horrendous -- 50%!!  Better to be cautious and not worry whether the IRS will catch something.  By the way, if you do mess up and miss one, you can appeal the imposition of the penalty.  Again, my experience is that they are pretty forgiving on the 1st slip up.  Not so much on subsequent oversights.