Can I write a stable MystCraft age that has dense ores?

By definition, the Dense Ores symbol adds instability, and instability leads to decay. There might be a way to remove the instability through cheating, but that is, as stated previously, cheating.

Your best bet would be to have a chicken, cow, and sugarcane farm, so that you can make the books that you need. Once you get Dense Ores symbol, you can learn the symbol in the Writing Desk, and then write the Dense Ores symbol into any age you want to.

You can counteract the Dense Ores symbol by adding the Charged symbol to the age a few times. You will likely get some debuffs as well, but Decay is now limited to the more unstable ages, and the less unstable ages are generally safe except for the potion effects.

I don't know if you'd count this as cheating or not, but you can view an age's instabilities by opening its agedata.dat file with NBTEdit. The agedata file is located at (save name)/data/agedata_(age ID).dat.

If you don't see one of the Decays listed, it's safe to inhabit.