Please follow the below steps for display Date of Birth field in front.
1. Login in Magento 2 panel
2. Go to Stores => Configuration => Customers
3. Under customer => Select Customer Configuration option
4. Expand Name and address options tab
5. Here you can see that field "Show Date of Birth"
6. If you select "Required" option then it will display in registration form with mandatory field. If you select "Optional" option then it will display in registration form with non-mandatory field. So As per your requirement, please select either required or optional option.
7. Save the configuration.
That's it.
Now you can see this Date of Birth field in registration form.