How old do you believe our planet is?
We actually do have the one planet (Venus) in our system which actually is ballpark for some sort of a 6000 - 20000 year age like you'd get from Bible chronologies and Venus LOOKS like that: 900 F surface temperatures, major thermal imbalance, major upwards ir flux, 90-bar CO2 atmosphere, statistically random cratering etc. etc. etc. Since Earth and Mars do not look like that in any way, shape, or fashion, you have to assume that they are much older than that, but not billions of years old.
Bob Bass, one of America's best mathematicians over the last century, once redid Lord Kelvin's heat equations for the Earth WITH a maximal possible figure for radioactive elements, and came up with an upper bound of about 200M years which I would take as an absolute upper bound. If I had to put money on such a guess I'd say more like a few hundred thousand to a few million years.
That is of course, viewing our planet as a pile of rocks. Our living world in its present conformation is likely more like five or ten thousand years old as per the Bible. For that matter there are easily recognizable pictures of known dinosaur types in Amerind petroglyphs and other art work and descriptions of dinosaurs both in Midrashim and in Amerind oral traditions so that you assume there were leftover dinosaurs walking around at a time just prior to the flood, but I would assume the real age of dinosaurs would be more like a few tens of thousands of years back.