Stacking 6 blocks at age 3 would be delayed milestone. Should be nine. I'm not trying to be confusing based on my above post, but 6 blocks is definitely age 2. 5-6 is acceptable for age 2. But in your defense in real life nobody really cares about this sort of thing anyway and assessing progression is based on a myriad of things, not exactly how many blocks a kid cares to stack.