I remember reading a blog about a minorwho went from Alaska to Tierra del Fuego by bicycle ALONE. It was fascinating. I did a 1500 mile tour alone when i was 17--from Washington DC to Windsor Ontario. I crossed an international border and had no problems.
As has been said here by others, GO FOR IT. It will be a trip you remember forever, and many of the lessons learned during your journey will serve you well. I am now a grandfather and would encourage any of my kids and grandkids to do something like this--once they have shown they are mature enough to deal with it. And maturity is not something that is automatic when you reach 18 or 21. I have known many 13 year olds that are more mature than 3 year olds.
train safe-