Our 18 month old kisses us when we ask her to. My question is, at what age we should tell her not to kiss everyone who asks her to?
I would say that it is not needed to wait. Contrary to @Alexander, I would not stop completely to ask for kisses, as my wife and me often do with our 4 years old girl. And also, as I do with my wife, and as she does with me. I think asking for kisses from someone you love is acceptable with childs as well as with adults.
The thing you can do is teach her that when she is asked for kisses, she does not have to kiss. She can refuse to kiss, and it is ok. Of course, this means that when she does not want to kiss you, you should be ok with it.
At 18 months, she is able to make choices.
When my daughter was 18 months old, she would always give us kisses when we asked. Now she is a little older and, if she doesn't want to give us a kiss when we ask, she doesn't. We didn't have to teach her, she just figured it out. Perhaps you should begin to worry about this if it becomes a problem with other people but I think she will figure it out for herself.
You should tell her not to when she starts kissing those you believe shouldn't receive that level of intimacy from her, and you should be an example of appropriate intimacy levels with others. You may want to encourage her to perform these social acts with those you do want her to have that type of relationship with - kissing grandparents and siblings might be acceptable depending on your culture and values.
Infants, toddlers, and young children thrive on physical interaction and acts of appropriate intimacy within their family relationships. Particularly prior to their ability to communicate verbally, physical social communication helps them understand their place in the family and the world, and their relationship with others.
When you encourage them to shake hands, high five, or kiss another person, you are essentially telling them what relationship they should have with that individual.
If they disagree, they'll generally let you know through actions or refusal. They may need to warm up to a person before they accept kisses and similar gestures.
Please note that in some cultures kissing, such as at the sides of the cheeks, is not reserved for high levels of intimacy or even close friendship. There are no specific, absolute guidelines for child development regarding such social graces. Decide what you want for your child, and teach them that.
Kids don't normally need explicit instruction from their parents in order to conform to the behaviors that are expected in their culture. They learn these things by example. For example, if your kid is growing up in France, she'll learn that it's normal for the male President to kiss male dignitaries on the cheek.
You don't say why this is a concern, so possibly you're worried that she'll be a victim of sexual abuse. The thing to realize here is that when children are sexually abused, it's almost always by someone they know well. Therefore your child will not be made any safer by telling her to refuse to have kisses with people she doesn't know well.