Custom Number fields can be added by clicking on the Plus button in the Field manager, on both Organization and Event levels, and selecting the option Number in the dropdown offered.
Planners can edit the name of the field (e.i. Age), label for the field visible to contacts and participants (e.i. How old are you?) and the category the field belongs to (e.i. Personal information).
Field can be further customized by selecting the minimum and maximum number of characters that can be used for this field, as well as what might be the default value for this field.