I think it would be fine to end high school at 16 as is done in Germany.
If you meant that Germany high school ends by default at age 16 for all students, then yes, the USA is radically different in that respect. Also, Germany's summer vacation is 6 weeks instead of USA's 12 weeks. Since Germany ranks ahead of USA in primary education standings, maybe they have the better system.
[1]http://statelaws.findlaw.com/florida-law/florida-compulsory-...
In the UK we have preschools for kids around 2 to 3 years old. They're mostly focused around play rather than formal lessons, but I think it helps kids to get used to a school-type environment and start to socialise with kids their age.