Age of user in Family Sharing

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 the Apple ID user is 19, have them check their own Apple ID:

… Update the birth date associated with your Apple ID - Apple Support

If the Apple ID is correct, I wouldn’t be too concerned about what Family Sharing shows.

One path for clearing various iCloud glitches: sign out of your Apple ID on your device, force restart, and sign back in.