No Welcome Window

Unfortunately I can't test this because I already upgraded mine, but the Reminders User Guide states "If you choose to upgrade later, click Upgrade (next to your iCloud account in the Reminders app) when you're ready to upgrade." Do you have such an option?

Maybe see if you can update to 10.14?

https://support.apple.com/kb/SP777?locale=en_US

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

Ah yes, that's correct! Running the newest macOS is also one of the best ways to keep your computer secure, so you should upgrade at your earliest convenience.