I've heard a lot of things and already a lot of good info in here. Nutrition and Training are the main things then add in acouple natural supplements and stay on that path. If you hit a plateau, then eat more and train harder. Although i have heard around 25ish is your prime time and then for some around 25-30 testosterone slowly starts going down hill and it will be harder to maintain and even harder to grow.
I've been consistently lifting since I was 18 and I am now 26. I've put on about 30 pounds of muscle in that time frame naturally and am wondering how much more I can realistically expect to add?
Only limited by genetics, everyone is different.... without your dna and family history no one knows this answer.
that is a question you should answer with the following questions:
1.how bad do you want it
2.what are your genetic and financial capeabilities
3.do you have the balls to sacrafice,lift and eat??
4.what you define as natural
and only you can determine the answers