There are possibly options other than guardianship to explore and discuss with your attorney.
If the circumstances are applicable, the Texas Estates Code requires the use of less restrictive alternatives to guardianship. Such alternatives include, but are not limited to, Surrogate Decision-Making as described in the Texas Health and Safety Code Sections 313.001-313.007. If the adult child has capacity, parents can be appointed in a Durable Power of Attorney under Texas Estates Code, Chapter 752 and Medical Power of Attorney under Texas Health and Safety Code Sections 166.151-166.155.
Contact an attorney to review your situation for proper advice.