Unpaid Taxes

Your parents are nearing retirement. Their house is paid for and they have accumulated other assets to aid in that retirement. They jointly owe income taxes for prior years or never filed a return. The IRS does not inquire as to unpaid taxes or unfilled returns. The years pass. They now receive social security and Medicare. Both of them are retired.
The later you address the situation, the harder the solution. If one or both die owing taxes, the responsibility for addressing the situation falls to the executor. Estate administration becomes more costly.