I am not writing the entire letter for you. Also, I do not know the format in which you are supposed to write. That would take long but I am definitely helping you. Take key words from the question itself. So, here are some points instead. Tell your "friend" how touched you were after meeting them and interacting with them. Describe the old age home and the people who were there to attend to their needs. Make sure to write about how nice the old people were there and how you felt bad for them. End on a good note, asking your friend to reply to your letter as soon as he/ she got time. That's all I can give you. I hope this information will help you write your letter.