Could guns (powder) and cars (gas) be common in a world too young...

Too expensive, illegal, considered unrighteous by sun worshippers who think everything should be solar. etc.

satyesu said:

...for large fossil fuel deposits? Would they if they could be? If both answers are 'yes', how do I prevent those things (cars, planes, guns) but keep things like radio, skyscrapers, and neon lamps (yeah, right)?

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Gunpowder doesn't require any fossil fuels. Coal is used, but in areas without open coal deposits, creating coal from wood has been done for centuries (maybe millenia). Guns only require some luck when experimenting with various chemicals, or alternatively a good knowledge of basic chemistry. If a culture has the latter, working out how to make gunpowder is pretty much inevitable. If they're still at the alchemy stage, they just might not have stumbled upon it yet.
Cars also do not need fossil fuels. Cars can run on many things, wood included. They're not exactly going to have the performance of a porsche, but that's not what a car needs to be useful anyway.
Radios, skyscrapers and neon lamps require a lot of technological and industrial background, it is impossible to have them without the basic knowledge that would allow you to work out gunpowder (and even more advanced explosives) and it is extremely unlikely to have them without internal combustion engines. And impossible to have them without at least other machines with equal performance. Steam powered cars with high-performance, lightweight steam engines that replace otto motors are a possibility of course. You're not going to be able to build a skyscraper without advanced machinery, you'll not even be able to create the necessary materials for it.
That being said, the obvious way out is to have something like magic replace the neon lights and the machinery, while having it work in a way that prevents magical guns and magical cars.

There is a simple solution.
We call it "renewable energy".
That's as much as I can say with the info you've given.
What's the technology level of said world? What resources are abundant? I'm pretty sure sufficiently intelligent life will always find ways to develop analogies to planes, cars and guns. It just depends on how much time they've had and what they have to work with.
Oh by the way: guns have nothing to do with fossil fuel.

Edit: errr yeah. Posted too late to be relevant. Ah well. Seconding Wark, I don't think you can exclude those things without resorting to contrived ideological reasons.
But that's fine, those have been used for far more absurd things in real history. Such as forbidding people to eat certain things at certain times, outlawing sexual relations between individuals that haven't been approved by a servant of the divine, etc.

Your problem with a world that young is having sufficiently evolved lifeforms.

beezle said:

Your problem with a world that young is having sufficiently evolved lifeforms.

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Maybe they were dumped there by advanced aliens. Who also told them they'd perform orbital bombardment, should they ever dare to develop guns/cars/planes.

Simply, you need fossils to produce fossil fuels. Likely energy sources would be based on mechanically recovered energy (think waterwheels, wind power, and tidal forces), and steam (assuming you've got combustibles like wood).

As has been pointed out, gunpowder doesn't require fossil fuels. It does require charcoal, but you can get that from trees.

Electricity (the stuff you need for radio and lights) is pretty easy to generate without fossil fuels. Earth's issue is that fossil fuels are incredibly convenient to harvest, high energy, and inexpensive. If one of the first mass extinctions hadn't occurred (488m years ago) it's possible that intelligent life may have evolved here considerably sooner than our own species. Fossil fuels would have been much harder to come by at that point. In that event, our technological development may have been slower, but I don't believe it would have been impossible for us to get to the point we're at today.

As a geologist, I think it would be very difficult.

If we use earth as an example, a young world is likely going to be very tectonically active. This means very high heat flows from the mantle upwards. High heat flows would likely bake any buried organic matter before that organic matter could spend time in the oil window and generate oil. The prospects for coal wouldn't be too much better.

However, there's flip side to high tectonic activity: there's fantastic potential for geothermal power to generate electricity.

Of course, your buildings will have to be built exceedingly robust to withstand all the earthquakes. So you can have a society of engineers and non-stop Star Trek marathons.

Edited to add: the earth's early atmosphere was mostly methane, so hey, maybe that's your energy source right there! Of course, you don't have oxygen to burn it in.

It would seem highly odd to me to keep some of the technology you want without also keeping plastic, and figuring out plastics would most likely assume gunpowder and gasoline.

I usually lurk but I have to correct a few mistakes...

Gunpowder doesn't require any fossil fuels. Coal is used, but in areas without open coal deposits, creating coal from wood has been done for centuries (maybe millenia). Guns only require some luck when experimenting with various chemicals

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Your talking about charcoal. Coal is a mineral that takes millions of years to form.

As has been pointed out, gunpowder doesn't require fossil fuels. It does require charcoal, but you can get that from trees.

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It also requires salt peter, sulfur, and salt.

Salt and sulfur are easy to find, and salt peter can be extracted from urine.

Quetzlquatl said:

Your talking about charcoal. Coal is a mineral that takes millions of years to form.

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I'm talking about carbon. There is no real difference chemically between coal and charcoal.

satyesu said:

...for large fossil fuel deposits? Would they if they could be? If both answers are 'yes', how do I prevent those things (cars, planes, guns) but keep things like radio, skyscrapers, and neon lamps (yeah, right)?

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I would say its highly unlikely that you can have skyscrapers ( ie . modern civilization) without modernized transport mechanisms. If you wish to avoid cars etc... you will have to come up with a superior mode of transport that renders cars useless.

ELMontague said:

Salt and sulfur are easy to find, and salt peter can be extracted from urine.

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Or bat guano

But there might be the chance that the world is lacking and evolved without salt or sulfur

I'm talking about carbon. There is no real difference chemically between coal and charcoal.

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Good point

Perhaps if in your world carbon only existed as a trace element? (due to a particular kind of geology, catastrophic event, sun type etc) Of course, that would require all kinds of changes to your world, non-carbon based life-forms etc...