No, I would not make my 12 year old sit in a booster seat. A backless booster is not in and of itself a life saving device and seat belts can still be improperly positioned when using one. A child's head should not exceed the height of the back of the seat when he is seated in the booster. Actually I think its the tip of the ears but I am not certain.
You may want to read this article that the authors of Freakonomics published:http://www.freakonomics.com/times0710col.php
They question whether car seats are any better than seat belts at protecting children over the age of 2.
I am in no way adovcating ditching car seats for young children but it is an interesting article and includes a bit of research that they did including a memo from the Institute for Highway Safety back in 2001 that "The Institute also is concerned that NHTSA's public recommendations for booster seats are getting ahead of science and regulations for child passenger safety. For example, the agency recommends that all children weighing 40-80 pounds and less than 57 inches in heightshould ride in belt-positioning booster seats (www.nhtsa.dot.gov/
people/injury/childps/boosterseat/CPS_Week.pdf). As a result, child
safety advocates around the country are promulgating these guidelines,
and many states now are passing laws. These recommendations, however,
are based on a fairly limited study conducted in the early 1990s in
which seat belt fit was evaluated for about 150 children ages 7-12
positioned in the rear seats of three different vehicles. Belt fit was
evaluated with the seat belt alone and with three different booster
seat models. Although this study provided an important demonstration
of the potential for adult belts to be poorly positioned, by itself it
does not support these global recommendations and laws."