To have the storage on 1 tab, but the display on another you'd use a Custom HTML field to substitute in its display. The trick for a date field displaying as age though is you need to use the formatted substitution and not the raw substitution. So you need to use the following usage.
[cb:userfield field="FIELD_NAME" /]
More on substitution usage can be found in the below tutorial. Note your field needs to be set to display on profile for the substitution to work, but it can be on a tab in a not shown on profile position.
www.joomlapolis.com/support/tutorials/107-use-cases/18353-using-substitutions-throughout-cb