If you’ve accidentally submitted an incorrect birth date (too young) on a Steam product page your access to the content will be blocked and there isn’t an option to reset this.
To circumvent this message (in Chrome):
– Go to the page displaying the error message or the page that show the birth date selector
– Open the Chrome menu and go to “More tools” > “Developer options” or press F12
– In Developer options go to the Application tab
– Search for the “birthtime” cookie, right click it and select delete. Close Developer options.
– Now enter a functional birth date and you’re good to go