Can parents legally force a 20-year-old out of the house in Texas?

Absolutely, the person in question is of legal age and the parent(s) can request he/she leave the residence and they do not need a specific reason for doing so. If the adult child refuses to leave voluntarily and the parent(s) so choose they can enlist the aid of law enforcement personnel. Please be advised that the young adult does have the legal right to take such personal property as belongs to her/him, including those items which were given or assumed to be given as gifts, and/or a vehicle titled in their name only. The person must be lawfully evicted, and cannot be just thrown out.