Pixiq

Wow, lot of questions! Here's a Cliff Notes version. First off, you really have to research the model release laws in your individual state. A lot of people don't realize that model releases are state specific. But if your work appears in another state, it needs to follow their rules. SO we recommend to follow NY's model release law, since following that will cover you in all 50 states. It very basically says get a signed release.
You should have a release to show your work of anyone from a senior or wedding shoot on a website. Ed the lawyer would probably say you have to. I'll say you should, but it wouldn't stop me for most images. If it's a shot that could embarrass someone, I'd definitely want a signed release.But if someone objects, I'd take it down without waiting. Also, a bride or groom can't sign off for anyone else, including each other. Adults have to sign for themselves. Bride Judy can't sign off for Uncle Joe. As far as age when someone can sign, refer to your state's laws.
Hope that helps.
Whew, my fingers are exhausted.
Jack