This works even if your birthday is tomorrow
=IIF(DateTime.Today<DateAdd(DateInterval.Year,DateDiff(DateInterval.Year,Fields!fechaNacimiento.Value,DateTime.Today),Fields!fechaNacimiento.Value),DateDiff(DateInterval.Year,Fields!fechaNacimiento.Value,DateTime.Today)-1,DateDiff(DateInterval.Year,Fields!fechaNacimiento.Value,DateTime.Today))