Difference between this and self in JavaScript

function Wilto() {
	var self = this;
	self.age = 32;

	setInterval( function constantBirthdays() {
		self.age++;
		console.log( "I am now " + self.age + " years old");
	}, 3000 );
}
Wilto();

function Wilto() {
	this.age = 32;

	setInterval(() => {
		this.age++;
		console.log( "I am now " + this.age + " years old");
	}, 3000 );
}
Wilto();