Why do younger people have to be completely respectful?

I know it's always good to be respectful, but why does everyone say respect your elders? Well, what if they're rude and mean. You shouldn't just sit there and let them talk like that to you. It's rude! No matter how old you are, you still shouldn't be disrespectful. So why do elders disrespect some younger people or belittle children like they're stupid? The only difference of older people and younger people is there only a certain age apart. Other than that, we have eyes, a nose, hair... like everyone else. I try to be nice, but these older people tell us that we need to treat elders with disrespect, but yet they disrespect us sometimes. Don't get me wrong. The elderly are nice, but why can't we speak what's on our mind? Freedom of speech. I have the freedom of opinion, and if I think your not talking to me fairly, I'm not going to talk to you nicely either. Expect that. If you don't give nice, you won't get nice no matter how old you are. I would like to talk about this with my mom and grandmother, but they just don't understand why it's not okay for elderly and older people to treat younger people with disrespect when the younger person is nicer. I try to tell them, no matter how old you are, you don't have to be rude, because you will get rudeness. I'm nice and try to bee, but when someone is rude to me, I won't let anyone talk to me like that. If you know what I mean, please talk about it thanks. btw im 13 years old and a girl who is very nicee =)