When do you allow your child to have junk food?

My friend recently just accidentally gave her 14-month old a taste of potato chips. Haha.. She was having a small pack herself while her husband was playing with their daughter. The next time she knew, her husband brought their girl over and baby wants to have whatever mummy is having. She had no choice but to give her baby a small piece. Needless to say, her baby loved it. She is still trying to hold back on giving her more and had hidden all packets of chips. Still, she think it is just a matter of time. Personally, I believe moderation is key (though I practically grew up on chips.. opps!). Growing up, my parents allow me to have my chips as long as my regular meals are not interrupted. ;)