Legal Question in Family Law in Texas

"Keep" their child? Probably not, but I'm not all that sure what you mean by that. If you mean keep the other parent from having custody of them, no, because at 18, neither parent has any legal right to control where the child lives. At 18, the child is not a child anymore and no one has custody anymore, so they can choose to live with mom, dad, grandma, their best friend, alone, with a girlfriend/boyfriend/husband/wife, or in another city/state/country if desired. The same logic applies if it's the child himself or herself wanting to move out. The traditional way to keep them at home is to keep supporting them, and point out you won't do that anymore if they don't stay at home (though there is actually a legal responsibility to support your kids past 18 if they're still actively enrolled in high school and working toward a degree, I've never personally heard of that duty being enforced by a kid who moved out and sued mom and dad for support). The only circumstance I can think of in which a patent could legally "keep" a child past 18 is if the child is physically or mentally handicapped to the point of truly being unable to manage at all independently, in which case the parent needs to petition the probate court to establish a guardianship for the child.