I'm a size zero and fit into size 24 Paige jeans, I am so petite I won't even fit in size 24 Seven For All Mankind jeans or certain brands, I only fit in brands that are made extra snug like Paige and Hudsons. I can't say that I eat very healthy, for many years I ate loads of In N Out, pizza, ice cream, I basically live to eat. I just happen to be able to eat anything I want and not get fat, it's just genetics.
I am approaching almost 30 and I still fit in my size 24 jeans. They are getting a bit snug though so I probably should watch more what I eat. I for one am not fat at all and very very very proud of that!
All good genes! To be totally honest I don't work out either...

If I did I would get toned super fast I think...
I love being able to eat anything and not work out and not get fat. Life is very good!!
Only fast food I eat recently though is In N Out, sometimes Subway or Quiznos, or Rubios... I won't ever touch McDonalds though, that's like ratmeat...