Do stepchildren qualify for USAA even if they were adults when their parent married their stepparent?

USAA coverage is available to the children of a serving parent regardless of blood relation. So your stepchildren would qualify. USAA benefits are available to veterans and family. As long as it is a parent or spouse that has served the rest of the family is covered. USAA does not define family as a blood relation. One stipulation that may exist is whether you have adopted your stepchildren. According to some sources the stepchildren have to be legally adopted for qualification. Other sources go a bit further and state that the adoption has to occur while the child is a juvenile. According to information directly from USAA, eligibility can be passed down to children of marriage. The only instance where coverage would not qualify is in the case of siblings. If it is the sibling of a person who has served, and not the parent then said person would not qualify. Moreover, the basic qualifications that USAA has are as follows:
 

• Active, retired, or honorably discharged enlisted personell or officersof the U.S. Military
• Midshipmen, U.S. service academies, ROTC, Cadets, even ROTC scholarship holders.
• Individuals who parents served
• Former members
• Widows, widowers, or former souses who are unmarried

For information simply call an agent. They will be able to answer questions with more authority. The eligibility of your stepchildren will be for their own separate insurance plan. Since they are adults they have already aged out of family eligibility. Their qualifications only extend to their ability to sign up with USAA.