Yep, both questions depend (in part) on the age of the child.
1. Young children have proven to be credible witnesses at some times, but the children generally need to (a) know the difference between fantasy and reality (note that there have been a number of very young child witnesses whose memories may have been manipulated) and (b) understand that there will be consequences if they lie. Plus, of course, the child must be able to communicate, either through speech or sign language. A crying or babbling baby obviously isn't going to be a credible witnesss.
I believe the youngest credible witness in New York state was four years old. I'm trying to find a cite for that. Here is a cite for a case in the Philippines wherein a witness of four years of age had her testimony accepted (warning: it's a rape case, and not pleasant reading):
http://www.lawphil.net/judjuris/juri2001/may2001/gr_132364_2001.html
As for 2. That depends upon a number of things, not the least of which is the child's age, but it also depends upon the severity of the crime. Here's a cite re a child of 11 who was tried as an adult:
http://members.aol.com/digasa/stats33.htm
PS The age of majority is generally considered to be 18 years of age, at least in the United States.