Yes they do. Be sure to wear sunblock if you are are sun bathing in the nude because they are a really sensitive part of your body.
Alone without a guardian, probably 16 or 18, but with a parent there about any age over 12.
Yes you can, however it's not safe as you can't see the colour of your skin underneath and spray tans don't provide and SPF so therefore you would be more likely to burn.
Some people tan to feel beautiful, to not look so extremely white compared to many people who aren't nearly as white. Others tan to get rid of acne
How to Apply Make Up to Tanned Skin
Learn What a Tan Really Is
How to Keep Your Tan
How to Fix a Bad Spray On Tan