Who is required to wear seat belts regardless of their position in the vehicle?

The driver and all adult passengers (i.e. passengers who are at least 17 years of age) in a passenger vehicle are required to use safety belts if occupying a seat in a vehicle equipped with a safety belt. Children who are under 8 years of age and less than 4 - 9 in height (regardless of age) are required by law to be secured in an appropriate child passenger safety seat if occupying a seat in a vehicle equipped with a safety belt. Children who are at least 8 years of age but under 17 or who are under 8 years of age but taller than 4 - 9 in height must be secured in a safety belt. [Safety Belts; Chapter 14: Additional Safety Tips; Texas Driver's Handbook]