Sandra,
I can answer your questions pertaining to suzuki violin - I have been involved for more than 8 years now. My son who is now 12, started when he was about 3.5. My daughter, who is almost 10, got her first violin for her third birthday (I could not afford for her to play on mine :-)but we did not start formal lessons till she was close to 4 years old. It is a very time consuming activity and quite expensive. Right now I am looking into purchasing a full size for my 12 year old and I will probably have to spend $5,000 to $7,000, at the very least, plus another thousand for a bow and case. It's like he is getting a car at 12 :-)
Ania