These are only a few days old, definitely under a week. You can tell by how much feather development they have in addition to their fluffy down. Notice your yellow chicks have some wing feather growth? These may be little hennies, the girls' feathers often grow in faster than boys. You can always ask the store when the chick shipment was sent and when it got to them. Hatcheries send out day-old chicks and they may be 2 or 3 days old by the time they reach the store, and then maybe a few days more til you picked yours out. Also, if you know what breed you bought, you may want to look up pictures of each breed and what they look like at 1, 2, 3, 5, 7 days old etc. and try to figure it out. Hope this helps and happy chicken rearing! <3