In the United States, you can join the active military when you're seventeen. Pretty close.
The rest of the world ranges between fifteen and seventeen years of age.
In other countries, sixteen is considered quite 'adult'. In the UK, for instance, you can consume alcohol legally at sixteen, you can give consent for sexual activities, you can leave home and get your own place of living, work an 'adult' level job, etc. It's really only the USA that puts this hard line on eighteen as being 'an adult' whereas 16-17 is still considered a child. We Americans are unique in that belief.