Oh you turned 73? Do you have an IRA or 401K account? You have to start taking the RMD Required Minimum Distribution on retirement accounts. It will even start asking you if you took a RMD from a pension now. There were some bugs in the RMD questions earlier and people were getting wrongly hit with a penalty for not taking the RMD. So that's one thing you need to check. See if you have a Schedule 2 line 8 for additional tax from form 5329. Schedule 2 goes to 1040 line 23.
You got the underpayment penalty for owing too much on your tax return. Not for owing the IRS for any prior taxes or for not paying estimates. Although you might need to start paying estimates so you don't owe so much next year. Or you need to start having withholding taken out of your IRA and any RMDs and pension if you get a pension.
