You can join the US military at 18. An 18 year old dating a 17 year old is not a pedophile. What if the girl said she was 18 and then she reveals later she isn't?
The problem is US law is so fucked up that having sex with someone that's days younger than you can be illegal and punishable by labeling you as a sex offender for the rest of your life. Even if the other person had a fake id and lied to you!
Lots of countries have "Romeo & Juliet Laws" that decriminalizes sex with minors if both parties are within a few years of each other. For example, France has the age of consent at 15, and decriminalizes sex with under 15s if both parties are within 5 years of each other. There are caveats: the exception doesn't apply if one party has legal or factual authority over the victim, or are related.
The problem is US law is so fucked up that having sex with someone that's days younger than you can be illegal and punishable by labeling you as a sex offender for the rest of your life. Even if the other person had a fake id and lied to you!