OK… So this one has been \since the introduction of Paypal Express and the only way to fix it is to overwrite the Payapl Express review page.
You set your configuration to “Date of Birth” as a mandatory field in the configuration for sales / checkout.
This is fine if someone checks out through the regular checkout.
Customer clicks on Paypal Express on product page or cart, at no point do they get asked to fill in the Date of Birth required field.
Has been a headache with every upgrade for the past 4 years for me.
Please can someone take the time and code this into the Paypal Express review page for all future releases of thisof Magento.