There was some publicity recently because it was still legal for an adult to send a child of any age a text with graphic content because the content was in the form of text not photo. From what I remember, following an nspcc campaign, this changed.
It depends what you mean by sexting as talking about sex with a person of the age of consent wouldn't be illegal. Sending or receiving pictures would be because it falls under the child pornography laws.
Morally...
Personally why would you want to? I literally do not understand it. When you leave school, usually, you experience the feeling of seeing how young school kids are and seeing your own maturity mirrored in that. In your 20s what could you possibly have in common with a child? Who for all intents and purposes still is a child. They're still dependent on an adult and have very limited life experience.
4 years difference isn't a lot when those people are adults but when one person is 16 that is the difference between someone sitting their GCSEs and someone nearly graduating uni. Completely different levels of maturity and worldliness