If you know someone who's probably going to be a big star in the future, is it okay to register a domain name containing their first and last name? For example, if the person's name is John Doe, is it legal to register johndoe.com (and maybe put a site on there, try to earn some money) ? Can you lose the domain name later?
yes its legal to do so. many people have the same name so how would they regulate it? If they want it later they will have to buy it from you or find an alternative (there are tons of extensions so their bound to find one)
You can lost the domain at a later stage however you cannot lose the revenue you have gained. I would build a site, add amazon affiliates and AdSense as monetization options... and optimize it for the search engines.
That's very legal and you can keep all you've earned. You're free to register whatever domain you like. However, they might try to take it from you and you could lose it in some later point.
It's perfectly okay to register that person's name. What isn't perfectly okay is using it in such a way that you're making money off their name. It's up to that person to sue you if they feel like it and have the means to do so. Tolerance varies.
I wouldn't do it. Eventually they'll take the domain away from you.. When registering a name that isn't your own its clear that you're doing it for the wrong reasons.
I think the celebrity would have to trademark their name otherwise anyone can register as people have similar names.
Maybe not. But if you registered, say, Ford.com and used it to promote your car repair business, do you seriously think the car manufacturer can't sue you for trademark infringement? If you answer yes, then I suggest further reading on the subject. Beliefs don't matter when any applicable law and/or legal decision/s say otherwise.