I use the one you linked to for approximately age 7-10.
Staying in the same series (Faber), your other options are:http://www.amazon.com/First-Piano-Adventure-Lesson-Book/dp/1616776196/
(I use this for 5/6 year olds, and the one time I taught a 4 yo. If it's too easy for a 6 yo, they can either transfer to the primer you linked to, or after doing books A and B, then can go directly to level 1 and skip level C. http://www.amazon.com/Level-Lesson-Book-Piano-Adventures/dp/1616770783/
For approximately 11 year olds up to maybe 15? I use this http://www.amazon.com/Accelerated-Piano-Adventures-Older-Beginner/dp/1616779497/
Then finally for adults (15/16 and up) there's this http://www.amazon.com/Adult-Piano-Adventures-All--Lesson/dp/1616773022/
Oftentimes it really is a guess if someone is on a borderline age group. If I can do it without being awkward, I sometimes try to ask the parent things like "how does the kid do in school" or "how diligent are they at doing things like homework of their own motivation?" to get an idea of how hardworking the student *might* be at piano, if I have to buy their books before even meeting them, to help me guess if they should be on the older or younger level track.
They can always switch if needed, so it's not set in stone, (just the annoying $20-ish for new books!) One little anecdote; I had two girls who were friends start around the same time and they were about 10/11. For whatever reason, I put one in the "regular" track, and the other in the "accelerated older beginner" track. The entire several years I taught them, they ended up playing songs at basically the exact same level - the one in the regular level just moved faster, and the one in the accelerated level just moved slower!