I won't say she can do algebra until she can solve
2*n - 3 = 5
That's very similar to the questions on the end-of-5th test in our district, where it's not called Algebra until 8th or 9th grade. The exact question uses a table of "n"s paired with outputs, and asks for the formula that fits the table, where 2*n +1 is the proper one.
Going a bit off-topic, and IANAn educational theorist, but I think Bostonian's question would, in some circles at least, be considered a lot more challenging than the one you're mentioning. Yours can easily be done without an understanding of the "do the same thing to both sides" procedure (you ask "what's happening?" and it doesn't take much trial and error to see), whereas Bostonian's can't (so easily). All the same, if I took that definition of "can do algebra" then DS could do algebra at 4, before he started school! I'm not sure I really see that being able to do algebra should count as a milestone, however we define it - it depends too much on what algebra you mean!
