I used to play with my DS with the refrigerator number magnets and make the numbers jump back and forth on either side of the equals sign (we called it "making the numbers do tricks"), showing how they would change and he thought it was hilarious at age 2-3. At 5 he was very comfortable with concept of solving for x in simple equations (x + 2x or x -4 = 5 type things, although we tended to present in word problems around the dinner table) and I just don't see what is so different or difficult about algebra that it would be verboten until he is older.
Exactly, I've never understood that either. One of DS6's favourite games when he was 4 was "secret number sums", e.g. "The secret number is called x and the clue is that x - 4 = 5". He's actually found algebra one of the easier aspects of maths to absorb, e.g., when I look at syllabuses I tend to find that he's done much more of the algebra on the syllabus than of, say, the data handling. And it's a great area to be really secure on. E.g., he's been filling in a percentages gap just recently. The textbook (presumably because the author thinks of algebra as something that might scare readers off) presents each type of percentage problem separately - so that someone following that method has to keep remembering what number goes where in this particular problem type - but DS tends just to write an equation with an unknown for the thing he's looking for and then solve it, which means he only has to know what a percentage is, what the words of the question mean, and how to solve the equation, IYSWIM.
