Start on the groove. You may have to sacrifice the edge boards for leverage. Use a large pry bar and as you lift the edge of the board, kick it forward with the heel of your foot to release it from the next boards groove. Be careful and make sure you get enough of the prybar under the flooring to lift without cracking the tongue. Also - wear workboots. If you can get a system down where one person pulls up the flooring and the other removes the nails, you could save alot of time.
As for age , YES it does matter. The older the more brittle.
I've removed and installed alot of flooring over the years, and now i will only do it if its Heart of Pine. We did some today actually. They boards were 3 1/4 " Heart of Pine in various lengths up to 12 feet in a 1915 bungalow. It took 2 men 6 hours to remove and de-nail approx 750 sf. We're reusing it in the house. Inevitably some boards may crack - just try your best to minimize it.