They were probably sealed before or at installation, but the previous owner didn't do anything special to maintain it.
Ours are almost 7 years old, and I have never resealed them. But I use a granite cleaner that also has sealant properties to keep it maintained. There are some areas that probably do need to be resealed properly. Like around the sink, where water is splashed constantly, off and on all day everyday. But for most of it, it is still in excellent shape and doesn't need anything extra.
You can buy spray on do it yourself stuff, like Nancy mentioned. Just go to Home Depot or Lowe's and ask them what they recommend. You'll need clean soft cloths to apply it. If you have one of those electric things you buff your car with it will probably be easier and quicker to get it done. :)