My son linked in my family has an age of 5, and he is 22, how do we update the DOB for him?

As the age of your Son is stated Below 13, you Will need to follow these steps to change his DOB

To change a birth date to any age below 13,* you must be a member of a Family Sharing group, and your family organizer must approve the change. After you change a birth date to any age below 13, you can't edit it.* Learn more about Family Sharing. 

1. Update the birth date.
2. Follow the onscreen steps to send an email request to your family organizer.
3. Your family organizer will get an email with instructions to approve the request. 

I am understaning he is really 22 but was the age on his ID is 5, correct?

According to the support article: Update the birth date associated with your Apple ID - Apple Support

"If you need to change the birth date for someone below age 13 [age varies by country - see the article], contact Apple Support."

Click/tap on this link to contact support --> Contact - Official Apple Support

If he is really 5 but was set at 22, then to change a birth date to any age below 13 (age varies by country), the child must be a member of a Family Sharing group [ Set up Family Sharing - Apple Support ], and the family organizer must approve the change.

1. Follow the instructions in the article to update the birth date on an Apple mobile device or on the web.

2. Follow the onscreen steps to send an email request to your family organizer.

3. Your family organizer will get an email with instructions to approve the request.

Read the document in the first link for complete details.