1) Because 16 year olds work. Taxing 16 year olds without allowing them representation in government (by giving them the vote) is taxation without representation, something we fought a war over.
Beyond that, 16 is a reasonable age for anyone to start being and independent member of society. We do too much babying and expect people to live at home and be dependent upon their parents for far too long.
2) Again, because 16 year olds are legally able to work. In Atlasia, we outlawed the labor of persons under 16 with the Fair Labor Standards Act of 2013, which is why I don't advocate reducing the age of majority below it. If you're old enough to work, you should be old enough to enjoy all the rights and privileges (and duties) that come with being an adult.