If I pull out of my Rollover IRA, what are the penalties in Illinois? How will this affect taxes filed in 2026? What else should I know?

Illinois doesn’t tax retirement income so there’s no state penalty.  The IRS will tax the withdrawal at your marginal tax rate and assess a 10% penalty. 

just so it's clear..... by "under retirement age" means you are under 59.5 years old, you are unlikely to avoid the 10% penalty for withdrawing.  You also would owe the normal federal tax.  For Ilinois, there is no penalty for the state tax, but you would pay normal Illinois income tax.

Why are you withdrawing?  maybe you are eligible for a different exception to the 10% penalty? 

https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-exceptions-to-tax-on-early-distributions