Generally speaking it is not necessary to capitalize decades unless it occurs at the start of a sentence.
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You do not have to capitalize fourteen hundred dollars. The only time you would capitalize any word in that phrase is when the word "fourteen" was used at the beginning of a sentence.
There are 10.6 decades. Or 10 decades plus 6 years.
I see no reason to capitalize it. In general, you capitalize something if it is the name of a single, unique thing (for example, the name of a person or country); or if something is considered sacred or otherwise important. There are a few additional cases when you capitalize something, but I don't see how any of them would apply in this case.
70 decades
There are 70 decades in 7 centuries. This is because each century consists of 10 decades, so when you multiply 7 centuries by 10 decades, you get a total of 70 decades.