Okay I got a threat from a guy off of youtube. It said something about "Im gunna find out where you live and then im gunna kill you." Should I be worried about this? In all of his video's hes messing around with a knife. So do you think I should be worried about this?

I'm guessing it's spam...

I expect it's some guy that acts tough when he has the whole internet between him and the people he is threatening but I'd still report him to youtube and keep a record of what he said.
If he does it more than once I'd report him to your local police as threatening behaviour is a criminal offence even if it is done over the internet.

It's most probably ok but I wouldn't provoke him or put up with it if he does it again.

and why did he send you the threat?

My son who was 9 at the time once uploaded a video to youtube and claimed it was his to impress friends, when the original poster who was in his 20s saw it he sent a really abusive message to my son with terrible language so there are some very unreasonable people on youtube who are even prepared to pick on little kids!!

Yes and no.... Depends on whether or not your username is linked with real life information that could be used against you. For example, I am 22, but I always put my birth date as April 1st.

The only thing I have is my age is 15. Right when I saw that message I sent him another one asking why he would send me that.

Report him to Youtube and as long as you're not an idiot and list your personal information I wouldn't worry about anything.

-edit-
I wouldn't worry too much about him know your age.

Tell him to BRING THE PAYNE, but seriously don´t worry about that kind of thing, the internet was made for people with low self esteem who are to young to drink so they can feel strong, besides you live is the U.S. if he gets on your property just shoot him in the face (with a bebe gun, I´m against vioelence but this guy might need a little)

Report him. Without any personal information he can't do anything but youtube should ban the account if he's sending threats to people.

Hint for the Internet: Ignore anything personal.

Seriously, youtube comments generally are so pathetic anyway I don't know why anyone would take them seriously. As long as you've been smart and haven't put any personal details up you should be fine. Given you are 15, I'll assume you are part of the myspace crowd (or at least one of those social networking sites). Be very careful what you put up there too. While I doubt it is common, there have been people that have had various issues with Work/School due to stuff placed on myspace/facebook/etc...

Simply, treat the internet as if everyone on it is a stranger.

Go find him and teach him a lesson.
Nah jk... Just ignore that fool.