If I’m 15 years old and 5’8 and deciding to buy a NBA jersey would I be a youth large or youth?

Neither, youth sizes are for kids. An xl in youth sizes is size 16-18 with chest size 34"-36" and 120 lbs so unless you are an exceptionally skinny guy you need to be ordering from the adult sizes. I'm pretty sure they have a sizing chart to click on somewhere on the site.