Hi Raul, I have a young student 15, about his size that I have been training along with other instructors. He has some strength issues on long flights, 1 hour, and uses two hands on the stick at times. We are training in 2-33s and the stick can be a bit stiff. I think adding to that the nervous reaction of over controlling and the death grip on the stick has an impact on his endurance. I think with more stick time and comfort with his flying that will sort itself out.