A kid who just started to read puts a lot of effort in processing sentences. These efforts will decrease as he/she gets older and, at some point (around 12, 15?), will read as fast as an adult.
Is there an age when reading speed becomes stable enough to be considered as adult speed?
My guess is that it should depend on each individual (particular pleasure/difficulty to read, variable exposition to written material), but there should be an average measure. I searched the web for some references without success - maybe this question was answered long before the digital era.