At age 75, due to Required Minimum Distributions (RMDs), their taxable income will increase, pushing them from the current 12% tax bracket into the 24%, 32%, and eventually 35% brackets.
"once you get to age 75 and you have to start pulling those requirement distributions instead of getting to live in the 12% tax bracket that you guys are living in now all of a sudden now you have to jump into the 24% bracket which eventually turns into 32% bracket which eventually turns into 35% bracket"