I don't think there is any magic age for too young for radiation.
Having said that the last radiosurgeon I went to told me that he would do GK for me if that is what I wanted but he recommended microsurgery.
It was a combination of age, 43, and size of the tumor, almost 3.0 cm, that drove his advice. Since my tumor was right up against the brain stem that was a concern and the doctor told me that while GK is very, very precise there was no way to guarantee that only the tumor would be hit by the radiation. He was concerned that 20 years down the road he could not guarantee that I would not have issues if my brain stem did receive some radiation during the treatment.
I guess my point here is that there are at least two factors to think about: your age and the size of the tumor. The younger you are or the larger the tumor, the scale probably shifts to surgery. The older you are or the smaller the tumor, the scale probably shifts to radiation.
A third factor would be your overall health.
Best wishes,
Ken