More than one in five Mercer Island residents is 65 or older. The Census Bureau's 2020–2024 survey puts the island at 22.2%, against 14.0% for King County overall, the highest share of any King County city with more than 10,000 people. CMS doesn't publish city counts or how many people got non-renewal letters, but an older population means more residents for whom this decision is live.
The island next to the county
| Mercer Island | King County | |
|---|---|---|
| Total population (OFM, April 1, 2026) | 25,870 | – |
| Age 65 and over (ACS 2020–2024) | about 5,600 | 320,899 |
| Share of residents 65+ | 22.2% | 14.0% |
| Land area | 6.4 sq mi | about 2,116 sq mi |
| Hospital inside the city (DOH/CMS lists) | none | many |
| Medicare enrollees, July 2026 | county figure only | 368,813 |
| Medicare Advantage and other health plans | – | 53.6% |
| Original Medicare | – | 46.4% |
Sources: WA OFM; U.S. Census Bureau ACS 2020–2024 (B09020, margin of error ±533 for the 65+ count) and 2025 Gazetteer; CMS Medicare Monthly Enrollment, July 2026 (county-level only).
Two things fall out of this table. First, an older population means a lot of people here first qualified for Medicare before 2020, and that changes which Medigap plans you can buy (more on that below). Second, there's no hospital on the island in the state's list, so every hospital stay means crossing the water. Whatever coverage you choose has to work on both sides of I-90.
The dates, without the fluff
- Through December 31, 2026: your current plan still pays. Use it normally.
- By December 31: choose your replacement and it starts January 1, no gap.
- December 8, 2026 to February 28, 2027: your special enrollment period for a plan that's ending. January picks start February 1, February picks start March 1.
- March 4, 2027: last day of your Medigap guaranteed issue right (63 days after the plan ends).
If you let all of that pass, Medicare places you in Original Medicare on January 1. You'd have Parts A and B and nothing else: no drug plan, unless you receive Extra Help (CMS reassigns those members to a stand-alone drug plan), and no supplement.
Which Medigap plans you can buy depends on when you first became eligible
Washington's guaranteed issue law, RCW 48.66.055, is broader than the federal minimum. If your plan is ending and you move to Original Medicare, an insurer has to sell you one of these plans, cover pre-existing conditions, and not charge more for your health history:
| You first became eligible for Medicare | Plans available under the guaranteed issue right |
|---|---|
| Before January 1, 2020 | A through F (including high-deductible F), K, L |
| On or after January 1, 2020 | A, B, D, G (including high-deductible G), K, L |
Source: RCW 48.66.055(4); OIC, "Is your Medicare Advantage plan changing or leaving?"
On an island with this many residents past 65, that first row may apply to many readers. Plans C and F, which cover the Part B deductible, are closed to people who became eligible in 2020 or later, but if you were eligible earlier you can still pick them under this right. Whether they make sense next to the other letters is a price-and-coverage question for your own situation; the insurer only has to offer plans it currently sells to new customers.
When to apply: OIC says no earlier than November 1. Washington law requires insurers to accept an application sent before your old plan ends, with coverage starting the day after it ends. Apply in November or December and you can have a Medigap policy in force on January 1.
The trade-off to weigh: if you join another Medicare Advantage plan instead, you don't use this right, and once March 4 passes it is gone. Coming back to Medigap in a later year usually means answering health questions. The statewide guide walks through both roads side by side.
Getting across the lake to appointments
Mercer Island station is on Sound Transit's 2 Line, which now runs between Lynnwood and Downtown Redmond with stops including Judkins Park and South Bellevue. That gives island residents a train to both Seattle and Bellevue medical districts. A Regional Reduced Fare Permit brings fares down on participating Puget Sound systems; people 65 and older and anyone with a Medicare card can apply.
For people who can't manage buses or trains because of a disability, Metro's Access paratransit serves addresses within three-quarters of a mile of regular bus routes and Link stations.
Local and statewide help
The City of Mercer Island's Youth & Family Services and Parks & Recreation teams have put on an annual Senior Resource Fair at the Mercer Island Community & Event Center, which is worth watching for. For Medicare decisions this fall, though, go straight to the phone:
- SHIBA, the state's free, unbiased Medicare counseling program: 800-562-6900, 8:30 to 4:30 on weekdays. King County's local SHIBA sponsors were listed as not accepting appointments on October 26, 2026, with callers directed to 1-800-MEDICARE.
- 1-800-MEDICARE (1-800-633-4227), 24 hours a day, 7 days a week, except some federal holidays.
- Community Living Connections for King County: 206-962-8467.
Washington statute and federal Medicare rules shape everything above, yet your plan's own notice sets your dates, so confirm the details with SHIBA, 1-800-MEDICARE or a licensed agent before acting.
Questions from island residents
I was first eligible for Medicare in 2017. Can I still buy Medigap Plan F?
There’s no hospital on Mercer Island. Does that matter for my choice?
What happens on January 1 if I do nothing?
Sources
- WA OFM April 1, 2026 population
- Census ACS 2020–2024, table B09020
- Census 2025 Gazetteer
- CMS Medicare Monthly Enrollment, July 2026
- RCW 48.66.055
- OIC: Is your Medicare Advantage plan changing or leaving?
- CMS CY2027 enrollment and disenrollment guidance
- Sound Transit 2 Line schedule
- King County Metro: Regional Reduced Fare Permit
- King County Metro: Access Transportation
- City of Mercer Island: Senior Resource Fair
- OIC: Find a local SHIBA office
