This is not possible with the AutoCalc feature of Mindjet 11 for Windows.
However, you might use a workaround to show the age in months by setting the Task Info of a topic as follows:
1) Select a topic
2) Open the Task Info task pane
3) Set the duration to month(s)
4) Set the birthday as the start date (e.g. 18-4-2000)
5) Set the current date as the due date (e.g. 18-4-2013)
6) In this case the duration will show the age in months: 170 months
It's a bit cumersome, it would help if you would be able to show the task duration in years.