If platinum had been as common as iron, could there or would there have been a platinum age, or would it have been too hard to smelt (like bauxite) ?
Not hard to smelt, it actually acts quite a bit like copper. In fact that is the problem. It is too soft to really be useful like Iron, it has some interesting alloys but they are all with metals that are harder to work (iridium) or that work better with other alloying material (tin).
I suppose you could get a "platonz" age rather than a "bronze" age if platinum were more common than copper (i.e., "bronze" being an alloy of platinum and tin rather than copper and tin). But I don't see that being much of a change as the limiting factor in bronze was the availability of tin not the copper.
Also I suspect this would be minor except for any butterfly's that hit due to the concentrations of copper and platinum being in different places - but if all you did was swap copper for platinum I would say no impact at all...