yep, it's totally fine to use. in fact most powders for sale in indian markets are from crops that are as old as 2007. the only thing age really affects is the potency: henna from a 2009 crop will leave a much stronger dye deposit than than henna from a 2007 crop. so shikakai should be fine.
It should be fine. It won't damage your hair, as pookie said, it will just have less potency. For future reference, just keep your powders in the freezer. That way it they will keep for years.