San Quinn was like 14 or 15 when he laid down his first professional verse right? If so, thats the youngest in my opinion. No way anyone under 14... 13... should be even trying to rap. They should be refining their skills at that time.
Too old... hmm.... like 45-50... but we will see what the "current" old rappers (like 2 short, lynch, and them) do.