Have a look at this:
Working with Age: it's not the same as a TimeSpan![
^] - it doesn't do exactly what you want, but a simple modification to this constructor:
public Age(DateTime startDate, int years, int months, int days)
{
_startDate = startDate;
_endDate = _startDate.AddYears(years).AddMonths(months).AddDays(days);
GetAge(_startDate, _endDate);
}
Or calling it with negative values should do it.