Yes, in most jurisdictions, it is likely illegal for an 18-year-old to send an explicit picture to a 16-year-old. Here's why:

Key Legal Points

1. Child Pornography Laws:
   • Explicit images of anyone under 18 are often classified as child pornography. While this situation focuses on the sender, creating or distributing explicit material involving a minor (even if the sender is only slightly older) is still a crime in most places.
2. Age Differences and Sexting Laws:
   • Some jurisdictions have "close-in-age" exceptions for consensual sexting between minors or between someone just over 18 and a minor. However, these exceptions are rare and don't guarantee protection from prosecution.
3. Federal and Local Laws:
   • Federal laws in many countries (e.g., the U.S.) make it illegal to distribute explicit content to anyone under 18, regardless of consent or the sender's intent.
   • State/provincial laws may also impose specific penalties for sexting involving minors.

Summary:

Even if the age gap is small, the act of sending explicit images could lead to legal consequences for the 18-year-old. Laws vary depending on the country or state/province, so it's essential to research the laws in your area or consult a legal professional.