To calculate the average length of service for men between the ages of 21 and 30 in Excel:

1. Select a cell where you want to display the result.
2. Type the following formula into the cell:
   
   Formula:

   =AVERAGEIFS(company_service, gender, "M", age, "="&21, age, "<="&30) 

3. Press Enter to calculate the result.

This formula uses the AVERAGEIFS function to calculate the average of the values in the 'company_service' column that meet the following criteria:

- 'gender' is equal to "M"
- 'age' is greater than or equal to 21
- 'age' is less than or equal to 30

This formula should give you the average length of service for men between the ages of 21 and 30. To calculate the average length of service for women in the same age range, you can use a similar formula but change the "M" to "F" in the formula.

You can also adjust the age range and gender criteria in the formula to calculate averages for different groups. Just make sure to separate each criterion with a comma and enclose text criteria in quotation marks.