Use calculated values
Text processing doesn't support expressions. To deal with it and to allow keep complex expressions simple, we have introduced Calculated Values in version 1.1.8. Survey class has calculatedValues property, that contains the list of Calculated Value.
{name: "age", expression: "age({birthdate})", includeIntoResult: true}
When the question bithdate is changed, the calculated value is automatically updated it's value (read-only property).