You apply through the Cook County Assessor, and which exemption you are applying for decides whether you will need to do it again next year.
The Senior Exemption
Apply once. You will be asked for proof of age and proof that you own and occupy the home — a driver's licence or birth certificate, and a deed, tax bill or closing statement. Once granted in Cook County it renews automatically, so there is nothing to do in later years unless you move.
The Senior Freeze
Apply every year. As well as age and occupancy, it asks for total household income for the prior calendar year — $65,000 or less for the 2025 tax year, rising to $75,000 for 2026 — so it needs your income documents each time. A year not applied for is a year at the unfrozen value.
What to do about years already missed
An exemption you qualified for but did not receive is exactly what a Certificate of Error is for: tax years 2021 through 2025, paid back as a refund check from the Treasurer rather than credited forward.
Exemptions are not an appeal
Applying for an exemption lowers the taxable value. It does not challenge the assessment, and it does not use up your right to appeal — see the Senior Freeze guide for the qualifying dates, and check your parcel if you also think the assessment is too high.