tomUK:
It could be some congenital defect such as, e.g., a cavernous haemangioma. If undetected, such a thing could simply burst and start hemorrhaging. If it's cerebral, the hemorrhaging could possibly be fatal. And you wouldn't have any warning. Anyway, it could be described as 'natural cause', except you'd need to do an autopsy to find out about it. However, this is not at all what I think happened. It's just to say that there are rare examples where a 21 year old could suddenly die without warning due to natural causes.
About the autopsy:
I think it is a strange decision. I would think that unless the cause of death for 21 year old is abundantly obvious, an autopsy is pretty much standard procedure. I would even assume that an autopsy could be ordered if there is any suspicion of a criminal act (such as prescription drug abuse). Such a suspicion would obviously override any wishes of parents or other relatives.
So, assuming the guy was doping, do you think the parents knew about it? Or maybe the parents only suspect, and don't want confirmation by performing the autopsy (and possible leaks to the press)? What a horrible positions to be in.