tier specified in the following table shall increase, on a sliding scale in a linear manner, from the initial percentage to the final percentage specified in such table for such income tier with respect to a taxpayer of the age involved: In the case of household income (expressed as a percent of the poverty line) within the following income tier: Up to Age 29 Age 30-39 Age 40-49 Age 50-59 Over Age 59 Initial % Final % Initial % Final % Initial % Final % Initial % Final % Initial % Final % Up to 133% 2 2 2 2 2 2 2 2 2 2 133%-150% 3 4 3 4 3 4 3 4 3 4 150%-200% 4 4.3 4 5.3 4 6.3 4 7.3 4 8.3 200%-250% 4.3 4.3 5.3 5.9 6.3 8.05 7.3 9 8.3 10 250%-300% 4.3 4.3 5.9 5.9 8.05 8.35 9 10.5 10 11.5 300%-400% 4.3 4.3 5.9 5.9 8.35 8.35 10.5 10.5 11.5 11.5 (ii) Age determinations (I) In general For purposes of clause (i), the age of the taxpayer taken into account under clause (i) with respect to any taxable year is the age attained by such taxpayer before the close of such taxable year. (II) Joint returns In the case of a joint return, the age of the older spouse shall be taken into account under clause (i). (iii) Indexing In the case of any taxable year beginning in calendar year 2019, the initial and final percentages contained in clause (i) shall be adjusted to reflect— (I) the excess (if any) of the rate of premium growth for the period beginning with calendar year 2013 and ending with calendar year 2018, over the rate of income growth for such period, and (II) in addition to any adjustment under subclause (I), the excess (if any) of the rate of premium growth for calendar year 2018, over the rate of growth in the consumer price index for calendar year 2018. (iv) Failsafe Clause (iii)(II) shall apply only if the aggregate amount of premium tax credits under this section and cost-sharing reductions under section 1402 of the Patient Protection and Affordable Care Act for calendar year 2018 exceeds an amount equal to 0.504 percent of the gross domestic product for such calendar year. . (b) Effective date (1) In general Except