The ideal age to start Freestyle / Greco is dependant on the wrestler. My son has been wrestling Freestyle/Greco for 3 years. He started when he was in 4th grade. What I did was ease him into it slow. The first two years he wrestled and played baseball. This allowed him to get the feel for Freestyle / Greco. His 3rd year I made him make a choice between baseball and freestyle. I did this for 2 reasons. 1. by doing both sports they detracted from each other. and 2. I didn't have the time to keep up with both of his practices and his younger brothers. He opted for wrestling. And the freestyle / greco experience has paid off. Last year he decided to take the summer off to have some fun.
The difficulty we had, when he was younger and lighter, was finding practice partners for him to work with. Now that he is 14 and over 100 lbs hopefully that will not be so difficult.
So back to the original question of what age should they start. It comes down to the wrestler and what they want to do. Folkstye season is long and to add another 2-3 months of wrestling on top of that can be difficult. The one thing I like about the Freestyle season is that there isn't near the pressure put on the wrestlers. You do not have to worry about weight at the local tournaments. You show up pay your money, get weighed and they pair you up at that point.
Hopefully my post has helped and that I didn't ramble on too much. If any questions feel free to contact me.