That only happened with the aid of a machine which is unique as far as we've seen. However, in the future of Equestria, it's quite possible that machines would become more common which would possibly result in earth pony services going obsolete. However, the unicorns only harvested the food-the growing of it (which is like 90% of the process) was done by Earth ponies, so that scenario would probably not be the case.
An age spell is for the 'highest level unicorns', which would mean it would be very ineffective for a society as a whole.
Even if it were hypothetically canon if could have been an age spell for the 'highest level unicorns', which would mean it would be very ineffective for a society as a whole.
The comics and EqG aren't canon.
I doubt even something as good as MLP could make me like that excuse for a Star Wars film.
In Hearthwarming Eve, AJ grows a plant in her hoof instantly. And in Twilight's Kingdom Celestia says about Earth ponies who have had their magic taken away 'Without their strength earth ponies will be unable to tend the land'. We've also never seen any unicorn use magic to grow food or accelerate growth, and if they could then the alliance of the unicorns and Earth ponies would have no reason to exist. So whilst it is never directly shown anywhere that unicorns can't grow food and Earth ponies are the only ones that can, it feels pretty safe to say that this is the case from what we've seen.
I was thinking of that, too, but that didn't really escalate to the level of actual wars/fighting with weapons.
The thing is a society run by unicorns would starve to death since they wouldn't be able to grow enough food to sustain themselves.
What I think would be interesting is a 'race war' between each species to see who would win. I actually think the Pegasi would have the upper hoof in this, considering they could completely screw up the whether. We've seen Rarity manipulate the weather, but I think the Pegasi would be MUCH more effective at it.
1000x much better than all the EqG films combined with the force awakens! And by that I mean it was decent 7. 5/10 from me since they obviously put some effort into it