All skills are better developed early. Evo-devo cognitive neuropsychology is the field I deal with and, unofficially, I can say that there is a window before about the age of 12 where these "left brain" skills can really be developed proficiently. After about 12-13 the efficacy of training drops precipitously. So I agree with Evo in that cognitive/artistic skills should be learned at an early an age as possible.
I don't have any kids yet, but if I ever do, I know when and how to train them. The question is how hard you want to push them and in what capacity? I think it may be constructive to this discussion to say what I personally would do if I, say, had two newborns I wanted to see educated in an ideal fashion.
Again, and I think I may have dealt with this issue in an earlier post, but I would push hard on the so-called hard left brain skills between the ages of 4-12, especially 7-12. My research conforms largely to the Piagetian model of cognitive development so these stages relate roughly to the concrete and formal operation stages of cognitive development in that model.
It's too involved to go into in this short post, but there are neurodevelopmentary reasons why these are the age ranges where you want to have your kids buckle down and study. After 12-14 it's a whole different process of learning.
Back to the example, if I had two infants, I would put them through the general curriculum of the school but I would encourage (force) the learning of a harmony musical instrument such as the guitar or piano as early as possible. It doesn't matter which one, both would be better. The same thing with mathematical training. These are both left-brain skills and have an optimal window for development, as describe above. Right-brain skills don't seem to have the same rigid constraints on developmental timing but right-brain capacities are also not subject to the same quantifiable assessment that left-brain skills are.