No, your work will not become age-restricted simply by added an R-18 or R-18G tag to it. You will need to set the age restriction directly.
You are able to alter the age restriction settings after a work has been posted. The second you realize you've forgotten to adjust the settings, we ask that you please change them immediately.
Please understand that if we determine that your work does not have the appropriate age restriction setting, we may adjust the settings ourselves without giving you prior notice.