What is the best age to start Karate?
It is generally believed that the younger you start at something the better you will become. While this may be true, there are many cases where people have begun an endeavour quite late in life (relatively speaking) and supremely excelled. Many of the great Masters didnt start till their late teens and some not until they were in their twenties.
In truth it doesnt matter what age yo start training. All that matters is that you train sincerely. If you train in this manner, then what you get out of training will be proportianate to the energy and thought that you put into it.

Will I get injured?
All sports have their share of injuries. However Karate ranks as one of the lowest, lower than ice skating or even golf!
Does it hurt to get kicked in the nuts?
Oooooouuiiii helllll yeaaahhhhh!!!!! *Rolls on the floor clutching his crotch and pounds the ground with his fists!*