John Glenn is old. He was pretty old 20 years ago. All people grow old and decay in the end. Going into space for a short period of time can't change the aging process. If anything, gravity would help create less aging because of less pressure on the body. He was bound to grow old eventually.
Although Glenn's past space travel definitely has an effect on his health, I don't think it's right to speculate that he is currently hospitalized specifically due to his space travels. There could be many reasons for his hospitalization. Speculation like this only strengthens the argument for reducing space exploration and that is not fair.
John Glenn is an old man. Old men tend to have ailments. I don't think there is any correlation between his past space travel and his current health. His body just isn't working like it used to. This is what happens when people age, and is to be expected no matter what you have done in your life.
John Glenn's hospitalization is not due to past space travel. John Glenn is 95 years old. It is a blessing that he has lived this long and this well. If anything his space travel lead him to be more aware and on top of his health as well as able to afford the best care and kept him healthier longer than most people.