When i see an old homeless man i feel very sad, but not when they are young?

Ok, so heres the thing; when i see an old homeless man i feel so sad and pitiful and i always try and give them some money or i sit and talk with them for a while. however, when i see a middle age (25-50) homeless man i dont feel the same way. i feel annoyed that they ask for change and slightly disgusted by their appearance. Also, if im watching a movie and it shows an old man crying or happy or something, it sends shivers down my spine, but not when its a middle aged man. If you've ever seen "A Knights tale", the scene where Will is reunited with his blind father, and the father starts to cry with happiness, i burst into tears. heres the thing, im a 16 year old boy and i hardly ever cry. i have provbably cried 3 times in the last 5 years. what is wrong with me. why dont i feel the same way when someone younger is homeless, or when someone younger cries in a similar situation.