Roth Conversion at 66: First Full Medicare Year, 9-Year Runway, and the Social Security Decision

If you're 66 in 2026, you were born in 1960 — a cohort with an unusually long Roth conversion runway. Under SECURE 2.0 Act § 107, your required minimum distribution age is 75, not 73.¹ That gives you nine years of pre-RMD conversion capacity: 2026 through 2034. For a couple with $1.5M–$2M in traditional IRA balances, nine full years at $180,000–$200,000 per year can clear the entire account before RMDs arrive.

But age 66 also marks two important transitions. First, this is your first full calendar year settled on Medicare — the ACA income cliff is behind you, and IRMAA's two-year lookback is now fully in effect. Second, your Social Security full retirement age is 67, exactly one year away. Whether you claim at FRA, delay to 70, or are still deciding is the single biggest variable in how much you can convert per year for the entire nine-year window. A couple delaying SS to 70 has roughly $56,000 more annual conversion room than a couple that claims at FRA.

Your position at 66 in 2026

Born in 1960 and turning 66 in 2026, here is what shapes your Roth conversion math:

The Social Security decision — now on the clock

For the 1960 cohort, full retirement age is exactly 67.² You haven't hit FRA yet. Every month you delay past 62 earns additional credits; every month you delay past FRA earns two-thirds of 1% per month — or 8% per year in permanent benefit increase. From FRA (67) to age 70 is 36 months, adding 24% to your monthly benefit permanently.

A couple where each spouse has a $3,000/month FRA benefit: claiming at 67 yields $72,000/year combined. Delaying both to 70 yields $89,280/year combined — a $17,280/year permanent difference before inflation adjustments.

The Roth conversion angle is often underweighted in this decision. Social Security income isn't just taxable — it's subject to the IRC § 86 phase-in rule that makes each dollar of SS benefit worth approximately $0.85 in MAGI at typical conversion amounts.⁴ That means claiming $72,000 in annual Social Security adds approximately $61,200 to your MAGI before the first dollar of Roth conversion. Against the 2026 IRMAA Tier 1 ceiling of $218,000 (MFJ), claiming SS at FRA costs you roughly $61,000 in annual conversion room.

How SS timing reshapes your annual conversion ceiling

Assumes $18,000 in annual investment income (dividends/interest). Ceiling = IRMAA Tier 1 ($218,000 MFJ) minus investment income minus 85% of SS income. 2026 IRMAA threshold per CMS.⁵ Rounded to nearest $1,000.

Over the nine-year conversion window (2026–2034), the difference between delaying SS to 70 and claiming at FRA is roughly $61,000 × 3 years of SS-free conversion = approximately $183,000 more Roth funded at 22% marginal rate before SS starts. That's $40,260 in current tax, but converts $183,000 from an account that would otherwise be taxed at 32%+ at RMD age — a likely lifetime tax saving of $18,300 or more on just that tranche.

The IRMAA constraint in your first full Medicare year

This is your first full calendar year on Medicare (you enrolled in 2025 at age 65). The two-year IRMAA lookback means your 2026 conversion will set your 2028 Medicare premiums. Going forward, every conversion decision has a two-year delayed Medicare cost.

The 2026 IRMAA tier table for Medicare enrollees:⁵

IRMAA surcharges apply per person. A couple that crosses Tier 1 by $1 pays an extra $2,296/year ($1,148 × 2). The planning rule: convert to $218,000 MAGI exactly, not $218,001.

Nine years of capacity — what it looks like for your IRA

For the 1960 cohort, the nine-year pre-RMD window is the longest available to any currently-retiring age group. Compare:

The jump from the 1959 cohort (6 years) to the 1960 cohort (9 years) is the largest single-cohort increase in the series. Nine years at $200,000/year is $1.8M in conversion capacity — enough to clear a large IRA entirely before RMDs begin.

Worked example: Sandra and Mark, age 66

Sandra and Mark are both 66, born in 1960. They retired at 63 and have been living on portfolio income. They have not claimed Social Security — both plan to delay to 70. They file MFJ.

• Traditional IRA balance: $1.7M
• Annual investment income: $18,000 (dividends, interest)
• Social Security: not yet started; combined FRA benefit ~$66,000/year ($3,000/month each)
• No pension, no other ordinary income
• Medicare enrolled since 2025 at age 65

Annual conversion ceiling (SS delayed)

With SS delayed, their income floor is $18,000. IRMAA Tier 1 ceiling is $218,000 MAGI. Annual conversion room: $218,000 − $18,000 = $200,000/year.

Federal tax on a $200,000 conversion (with MFJ standard deduction of $35,500 for ages 65+):⁶

Taxable income = $218K MAGI − $35,500 standard deduction (MFJ base $32,200 + $1,650/person age 65+ × 2) = $182,500. Brackets from IRS Rev. Proc. 2025-32.⁶ State taxes not included.

The nine-year plan

By around age 72–73, Sandra and Mark's IRA is effectively cleared — two to three years before RMDs would have started at 75. They pay approximately 14–17% effective federal rate throughout, versus the 24–32%+ they would face on RMDs had the IRA grown untouched to $3M+.

IRMAA timing: the two-year lookback at 66

Your 2026 income sets your 2028 Medicare premiums. For those planning to claim SS at 68 or 69 (not 70), the IRMAA picture in the SS-start year can produce a one-time bracket jump: the year SS starts, SS income and high conversion income appear together for the first time. Sequencing slightly larger conversions in 2026–2027 and reducing in the SS-start year mitigates this risk.

What a specialist advisor models at 66

• Social Security timing scenarios (claim at FRA vs. 68 vs. 70) and how each reshapes annual conversion capacity over nine years
• Year-by-year IRMAA impact, including the one-time jump in the SS-claim year
• Whether to fully clear the IRA before RMDs or leave some balance for QCD donations after 70½
• State tax implications of large annual conversions (some states have IRA/pension exemptions)
• Taxable account sequencing alongside conversions to manage MAGI precisely

Sources

1. IRS: Retirement Topics — Required Minimum Distributions (RMDs) — confirms RMD age is 73 for those born 1951–1959 and 75 for those born 1960 or later under SECURE 2.0 Act § 107.
2. SSA.gov — Full Retirement Age by birth year (1960: FRA = 67)
3. IRS Publication 590-B — IRC § 408A(d)(3) per-conversion 5-year penalty rule applies only under age 59½
4. IRC § 86 — Social Security benefit taxation ($32,000/$44,000 MFJ thresholds; 50%/85% inclusion)
5. CMS: 2026 Medicare Parts B and D IRMAA tiers — Tier 1 MFJ $218,001; per-person Tier 1 surcharge ~$1,148/yr
6. IRS Rev. Proc. 2025-32 — 2026 tax brackets (22% MFJ ceiling $211,400) and standard deduction (MFJ base $32,200; age 65+ additional $1,650/person)

RMD age per SECURE 2.0 Act § 107. IRMAA thresholds from CMS 2026 fact sheet; based on 2024 MAGI. Tax brackets and standard deduction from IRS Rev. Proc. 2025-32. SS taxability thresholds per IRC § 86. Values verified June 2026.