I believe it could be too early, if it's done in the wrong way. If you make it a stress and you must pass the test, when the child is too young... that's the wrong approach. However I feel I have found a great approach. Of course when to start this with your child, it should be handled individually to each child.
There is a free service from the SAT collegeboard. I signed A1 up for the daily questions, and I signed up to get them too. Each day he will read them, and try to do them himself. Then he will let me know if he got it right, and on what try.
While A1 just turned 13, he loves knowledge, he loves a challenge and he loves facts. I figured this would be something he would love, and well I was right!! I made it no stress, nothing to worry about. I told him if you don't know, simply make your best guess, then use the help section to get help, and always read the sections that explain the answers. This will not only help him prepare for the SAT, and test taking, but it will also help us see where we have slacked, or where we need to put some extra effort. It's a win win situation, considering he loves it!
I figure later on we will purchase a book, and do some of the practice test, but for now we are just working on test taking skills, and seeing what we need to do to help him prepare.
I also found an ACT question of the day program. This one does not get sent to your inbox, you have to visit the site daily. However this is a large difference in these test, and it all depends on what school you want to attend. That being said, and the fact A1 loves facts, questions and quizzes, he will be doing these ones daily too. If it becomes too much and I see, or he complains I will change it to ACT one day SAT one day.