In Texas, riders can be as young as six years-old, as long as they are supervised by an adult rider. In fact, all riders under the age of 14 must be directly supervised by a parent, guardian, or another adult who was authorized by the parent or guardian. Kids under the age of six can't ride an ATV on their own. Passengers are not allowed on ATVs unless it was designed by the manufacturer to carry passengers. There are also some age-based limitations on the size and power of an ATV. Riders below the age of 12 should never ride an ATV that's 70cc or above. Young riders between the ages of 12 and 16 should ride ATVs that are between 70cc and 90cc. Riders age 16 and over can safely drive ATVs that are 90cc and above. Regardless of age or ATV size, all riders are required to wear helmets and eye protection. Appropriate footwear is also strongly recommended.
Check out these Houston, TX ATV/OHV resources: