There is a National Latin Exam. They have an exam for six different years or levels. My husband was fortunate enough to take the first year exam while in high school, because his was one of the few high schools offering Latin. He attributes much of his ability to think critically and learn easily to his study of Latin. Students of Latin generally score higher on standardized tests than those taking other foreign languages. Memoria Press has some great articles on the value of learning Latin if you'd like to learn more check out their newsletter. Here is a link to one of their articles.
In order to be eligible to take the first year exam, the student must be enrolled in a Latin I course without having taken other Latin courses. We of course want our children to do well on that test and we want them to learn Latin well enough that they will be able to read some of the great books in the original Latin. In order to take all six national tests, they need to start Latin I no later than 7th grade. Henle Latin is our curriculum of choice. We have books for all 7 of our children. My husband bought them so that we'll for sure have them. He bought them for the the twins as soon as we found out we were having twins. So in order to learn grammar before we start first year Latin, we are having our children study Greek.
The curriculum we love is called Elementary Greek from www.opentexture.com My oldest is now on the 3rd year book. We are really focusing on the Greek now and moving through the 3rd year much more quickly than we did the first 2, so that she will be ready to start Latin I in September. My second child is finished with her workbook of year 2 but I don't think she is quite ready to move on. She is very bright though, so I think if I work with her enough she may also be ready to start Latin in the Fall also. We'll see, if not she'll certainly be ready to start the following year. I may push her hard this year because it will be easier to teach the 2 girls together. Time will tell.