Question: In age 1, the only resource you buy are the grey ones, because: in age 1, the brown ones produce only one resource at a time, but in age 2 they produce 2. This means you profit more from buying brown cards in age 2 than 1. So it costs a coin, big deal. No matter what, age 1 or 2, when you buy a grey card, it produces one item. This means when the worse cards are in play, you should buy the ones that will be the same.

Note: It also helps to choose your playing style early right?

Answer: 
The analysis is technically correct, but practically kind of wrong.

Every choice of card in 7 Wonders comes with an *opportunity cost*, which is to say, "What are you giving up by choosing this card instead?"

It is true that the brown resource cards in Age 2 give you access to twice as many resources as their Age 1 counterparts, for the cost of a single coin which is usually not huge as far as things go. However, if you decide that this means you should ignore the brown cards in Age 1 and just wait until Age 2 to get them, then:

1. You run the risk of never getting the resources you will need, especially if your opponents are paying attention and carefully deprive you of key cards.
2. You spend less of Age 2 building more useful cards (e.g. Science and Military cards, especially the ones that enable you to build stuff in Age 3).
3. You potentially have to spend more coins building things in Age 1, if you're relying on neighbours to provide the resources.
4. Conversely, your neighbours will not be buying resources from you, denying you a source of income.

It's easy to be flippant and say "It all depends on the cards", but really that's the point - 7 Wonders is a game of making lots of decisions, and there is rarely a singular strategy that you can follow blindly to success.
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> This means when the worse cards are in play, you should buy the ones that will be the same.
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That doesn't make sense. If they're worse, then they aren't the same. 

Age I brown cards, Age II brown cards, and Age I silver cards are incomparable; no one is clearly better than the other. Yes, Age II brown cards give more resources than Age I brown cards, but an brown card obtained in Age I can be used to buy things for two more ages, while a brown card obtained during Age I can be used to buy things for one more age (and if you buy military in Age I, that military gives you benefits for all three ages). Silver cards can be used to buy high-value cards, but those cards don't show up until later ages; a silver card obtained in Age I can't be used to buy anything in that age. 

Your argument, which you aren't articulating completely explicitly and so I have to read between the lines a bit, is that if you first have a choice between one resource of Type A or one resource of Type B, and then later a choice between two resources of Type A or one resource of Type B, then you should first choose Type B and then Type A. While there are some valid intuitions motivating that reasoning, it is not a conclusive. One motivation would be the assumption that the game makers designed the game to be balanced, so if they give you a choice between one of Type B and two of Type A, then one of Type B must be worth two of Type A. But the very fact that you think there is a clear strategy means that you are implicitly accepting that it *isn't* completely balanced. Furthermore, you are assuming that the relative worth of the two types of resources are the same in both ages, so if one silver resource is worth two brown resources in Age II, then it must be worth two in Age I. But as I argued above, that argument does not hold, because there are things available in later ages that require silver resources, and thus silver resources are worth more in later ages.

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> So it costs a coin, big deal. 
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You can't spend coins on resources unless your neighbors have those resources.