You are not permitted to make a regular traditional IRA contribution for the year that you reach age 70½ and any year thereafter.  It doesn't matter that you did not reach age 70½ in 2018 until after you made the traditional IRA contribution.  If you reached age 70½ in 2018, any regular traditional IRA contribution that you made for 2018 is an excess contribution.  If your modified AGI is low enough to permit the same amount of Roth IRA contribution and you have sufficient earned income to support an IRA contribution, you can ask the IRA custodian to recharacterize your traditional IRA contribution to be a Roth IRA contribution instead.  Otherwise you'll need to ask the traditional IRA custodian to make a return of excess contribution from the traditional IRA.