Texas allows >18 if applicant is member of the military, veteran or honorably discharged.
I think that's the reason WA doesn't allow reciprocity, but, maybe not.... WA specifically says that they honor the SD Enhanced license, but, only for those 21+. If age was the only reason for declining TX reciprocity, they could simply have the same age restriction they have with SD...
https://handgunlaw.us/states/texas.pdf
Age Requirements:An applicant must be 21 years of age to submit an application for a TexasHandgun LicenseORmust be at least 18 years of age if the applicant:
1.is a member or veteran of the United States armed forces, including a member or veteran of the reserves or national guard;
2.was discharged under honorable conditions, if discharged from the United States armed forces, reserves, or national guard