I have a house that is 30 years old and on a slab. I would like to finish the slab floors. Is there any way to do this at this time, or does it need to be done when the slab is poured?

There are several good products available for topcoating existing concrete. Do a search online or contact a local contractor specializing in this type of work. I assume that your situation has no major cracks and that you are looking for a "real concrete look" appearance. If neither, you should consider an epoxy coating that would conceal hairline cracks and give you a lighter color appearance. You have a cheaper do-it-yourself choice here, since any paint store will have this type of product. Be sure to think about the exposed edge if this is a patio or carport situation. You need a neat appearance where the new coating ends at the perimeter. You might also gice a thought as to how to trim-out the junction with you adjacent wall surface. I would consider installing a rubber or vinyl base ,similar to commercial work. These come in 4 inch and 6" heights. Wood base is also preferred but costs more, and requires finishing.