In Florida, it is illegal for an adult (someone 18 or older) to have sex with a minor (someone younger than 18), even if the sex is consensual. Those who break the law have committed statutory rape.

What happens if an 18 year-old gets a 15 year-old pregnant?

A 15 year-old boy can be arrested and charged with having sex with a 15 year-old girl. If the girl is under 18, sex with her by any man, juvenile or not, is statutory rape. However, this crime is rarely enforced. Otherwise, probably 90% of high-school kids would be in juvenile hall or jail.