|Cousin Kevin- Adorable? Yes! In my public tree? NO!|
Well the first thing to understand is that most professional genealogists won't list a living person in a family tree. This is a pretty good rule for us amateurs too. The thing is, when you add information about a living person to something that may be viewed by others, some of them possibly not even related to you, you want to make sure that there is no way their identity can be compromised or stolen. There are a few ways to go about that and the best way is to just keep them off your online tree!
I do have a list of living relatives and their current information, but I try to keep that off my online tree so that I am sure their information is safe and uncompromised. Ancestry does have a built in feature for protecting the living by not showing any information about them. Most of the time it works. I have noticed that adding a photo to a living person does allow others to see a thumbnail of the photo in their hints pages. All it takes is a copy/paste and that photo is theirs now. And if you share your tree with others, they can take anything they like and make it as public as they feel it should be. So use some common sense and either don't share your tree, or keep your information of living relatives out!
If you simply must add a living person (maybe you don't want papers that you can lose floating around), there are two precautions you should take. First, tell the relatives that you are adding them and what information will be available. Make sure it's okay with them. Some people are less open than others and will not want all their information out in the ether. I have a private tree with living relatives on it that I use for research and to connect branches that sometimes connect to other parts of my tree. I don't share this one, and I put only the barest information on it. Just enough to make my connections. Second, don't add pictures! Like I said, the thumbnail can still be viewable, even if the actual picture isn't. Just play it safe and add them to a file folder on your computer for the individual. Not everything needs to be online.
As always, I am going to remind you that once it's on the Internet, there is no going back. Don't put anything online that you don't want found. No matter how secure you think a website is, there is always the possibility that it can be compromised.