I am usingused an 18v18V impact driver with over 200Nm torque on mine after a good soaking in WD 40 and they still won't budge. TheMy bike is only 18 months old!
The impact driver and 6mm hex did not shift it and my open-ended 15mm spanner started to round off the flats.
I solved the problem by dismantling the pedals off the shafts and got a ring spanner on the pedal nut and blocked the crank against the frame with a block of wood. Then my putting my heel on the spanner and applying all my weight in a downward force I managed to crack them off. I was replacing the pedals anyway but beware if you dismantle the pedals the bearings will probably fall out.