This is the relevant information: Texas and Federal law only regulate the ownership of all firearms to 18 years of age or older, and regulate the transfer of handguns to 21 years or older by FFL dealers.
This justifies what answer for what question? Q & A:
Passage: Texas has no laws regarding possession of any firearm regardless of age, without felony convictions; all existing restrictions in State law mirror Federal law. A person of any age, except certain Felons, can possess a firearm such as at a firing range. Texas and Federal law only regulate the ownership of all firearms to 18 years of age or older, and regulate the transfer of handguns to 21 years or older by FFL dealers. However, a private citizen may sell, gift, lease etc. a handgun to anyone over 18 who is not Felon. NFA weapons are also only subject to Federal restrictions; no State regulations exist. Municipal and county ordinances on possession and carry are generally overridden (preempted) due to the wording of the Texas Constitution, which gives the Texas Legislature (and it alone) the power to "regulate the wearing of arms, with a view to prevent crime". Penal Code Section 1.08 also prohibits local jurisdictions from enacting or enforcing any law that conflicts with State statute. Local ordinances restricting discharge of a firearm are generally allowed as State law has little or no specification thereof, but such restrictions do not preempt State law concerning justification of use of force and deadly force.

Question: Based on this passage, at what age can i buy a gun in texas?
18 years of age or older