Do I have to have ID with me at all times?
No you aren't required. I like to have my license on me all the time though, if I ever need to have "proof of myself"
yes, you do.
No, but it's preferred as it proves you are your age. Many stores and people ask for it (police officers, too).
It's best to
You shouldn't need it all the time. But it's probably best if you do have it with you
Yes. You should always carry some type of identification with you. In case of an accident, etc. people who assist you need to know who you are so please get in the habit of carrying it on your person.