For many low-income Medicare beneficiaries, Medicaid provides important supplemental insurance that covers out-of-pocket costs and additional benefits. We examine whether Medicaid participation by low-income adults age 65 and up increased as a result of Medicaid expansions to working-age adults under the Affordable Care Act (ACA). Previous literature documents so-called "welcome mat" effects in other populations but has not explicitly studied older persons dually eligible for Medicare and Medicaid. We extend this literature by estimating models of Medicaid participation among persons age 65 and up using American Community Survey data from 2010 to 2017 and state variation in ACA Medicaid expansions. We find that Medicaid expansions to working-age adults increased Medicaid participation among low-income older adults by 1.8 percentage points (4.4 percent). We also find evidence of an "on-ramp" effect; that is, low-income Medicare beneficiaries residing in expansion states who were young enough to gain coverage under the 2014 ACA Medicaid expansions before aging into Medicare were 4 percentage points (9.5 percent) more likely to have dual Medicaid coverage relative to similar individuals who either turned 65 before the 2014 expansions or resided in non-expansion states. This on-ramp effect is an important mechanism behind welcome mat effects among some older adults.