A young rascal, miscreant or swain. Can also be used as an affectionate term for teenagers or duogenarians. A yoth under that age, may be called a "yofthette"; a "yofthess" can be a female yofth.
A term common in Nottinghamshire and Derbyshire.
"I had an annoying bus journey in today, there was a couple of yofths playing music loudly"