Health Savings Modernization Act of 2018 This bill modifies the requirements for tax-favored health savings accounts (HSAs) to: allow the high deductible health plans required for an HSA to provide certain coverage without a deductible, allow HSAs to be used for the medical expenses of adult children who are under the age of 27, allow excess payments of the premium assistance tax credit to be paid to the taxpayer's HSA, modify the limit on out-of-pocket expenses for high deductible health plans, and increase the HSA contribution limit to the amount of the limit for the sum of the deductible and out-of-pocket expenses for a high deductible health plan. The bill also eliminates provisions in the Patient Protection and Affordable Care Act that restrict the individuals who may purchase catastrophic health plans. (Under current law, the plans are limited to individuals who are younger than 30 years of age or who have a hardship exemption.)