Hey dude,
I went to Japan for 2 months by myself last winter, I was 19 at the time. As for credit cards, if you stick to major cities and go to the big malls surrounding the train stations, all the stores within those malls will accept visa/Diners club.. I didnt see much mastercard. Its pretty rough finding places that take credit in smaller cities though.. even in Nagasaki, which is moderately big, I was hard up to find a machine outside the station. Your best bet is to use visa around central stations(shinkanen stations) and to withdraw and carry cash elsewhere.
Dont worry about getting mugged... your more likely to just lose it!
Oh, and if you need any info on the cheapest places to stay or eat, gimme a shout! I was pretty much living on a shoestring budget while I was there.