The green on the potatoes doesn't mean they're 'young'; it just means they've been exposed to air, I think; produces solanine, which is actually toxic if eaten.
Went to the source for your questions (Jacque Pepin and Julia Child's 'Julia & Jacque Cooking at Home'); here is what they say (A summary only - there are pages of description in the book!:
The age of the potatoes does seem to make a difference; I would think newer potatoes have more moisture in them, which would make it more difficult to puff up.
They recommend cutting the potatoes thinner than 1/8"; '3/16' is what is called for in the recipe, making sure the potatoes are uniform thickness.
Place the slices in a bowl of cold water, then right before you're ready to cook, dry them well on paper towels.
Heat the oil in a wide, deep skillet to a depth of at least 1 1/2 inches, to a temperature of 300 degrees. Heat a second skillet with at least 1 1/2 inches of oil to a temp of 350-375. Place the slices in the pan with the lower heat, being careful not to overcrowd. Shake the pan, very lightly, to move the slices around; slide in a rhythm that will keep the slices moving, but not excessively agitate the oil. Adjust the heat to maintain a 300 degree temp. Cook the slices for 5-6 minutes, until small blisters begin to form on the surface of the slices. If they're browning, take the pan off the heat for a few moments and allow the oil to cool a bit, then return to the heat.
NOTE: This is different from your technique: "The slices will stay in the 1st pan until you remove them in batches, for the 2nd frying."
With a skimmer, scoop up 4-5 slices from the first pan, allow the oil to drip off; lower the skimmer slowly into the hotter oil in the second skillet, allowing the slices to puff in the oil for a 2nd or two, then lift them out immediately.
NOTE AGAIN: "Some slices mya inflate evenly, some may bubble up a bit, and some not at all. " Put the slices that puffed fully on a parchment-lined cookie sheet for the final frying. Cover with a towel; at this point they can be held at room temp for several hours before the final frying.
For final frying, heat the skillet again to 350-375; place the slices in the oil; they will inflate again and start to brown. Keep turning them over and over and stir with the skimmer for about 1 minute, til they are crisp and evenly browned all over. Remove to a tray lined with paper towels. The finished puffed potatoes will not deflate. Sprinkle w/salt while warm.
Bon appetit!