There's definitely a difference. You can almost smell 'em. XD NOTE: NONE of my seasonal eggs turned "old," regardless of the fact that they're really much older than the others in my Hatchery. The eggs that I have that turned "old" are all the FREE eggs that we find on the planets anytime.
Fresh Kayoki Egg: http://images.rescreatu.com/pets/5/egg.png
Old Kayoki Egg: http://images.rescreatu.com/pets/5/eggOld.png
I can provide more images if you want, but I think you can just add "Old" to the image URLS to get the altered egg images. :)