The best way to approach discussing any aspect of puberty with your teen is to bring it up before the change is expected to happen. For boys, the average age at which the first faint whispers of a moustache will begin is 15 years old, with chin whiskers usually making their first appearance at 16 years. However, these are averages. More important to note is that the moustache will likely start to appear roughly three years after your teen first entered puberty. So don't wait until your boy is 15 to talk to him about facial hair if he began puberty at age eleven.
Also, don't wait for your teen to come to you about this. As the parent, you should take the first step and let your teen know what to expect when he starts to grow facial hair. Typically, the hair will first grow at the corners of his upper lip before spreading across the entire upper lip. Once the moustache has come in, the pattern of hair growth will usually spread from the corners of the upper lip to the upper parts of his cheek. From there, the hair will start to come in under the lower lip. Facial hair growth on the sides of his face and chin will be the final steps.
As for girls, some may see a growth or darkening of hair on their upper lip. Girls do not get as excited about this event as do boys. If your teen daughter does develop dark facial hair, let her know that she does have options to have it removed, such as waxing, laser or just bleaching the hair so it's less noticeable. If your daughter's facial hair seems excessive, you may want to ask your doctor about it as it may indicate a hormonal issue.