1. Because you are hurting someone.
You are actively hurting someone with any variety of disabilities. When you use this word you are hurting those who simply want to be accepted and understood.
2. Because you "growing up in a different time," is not an excuse.
I understand times change and terms that used to be normal now become hurtful and it would be a lot to keep up with. But, not really. I have great-grandparents who are in their 80's who have never said that word because they know it to be hurtful and not appropriate. There are no excuses.
3. Because using it is the same as using a racial slur.
A racial slur is offensive because it is a derogatory term used in times past that hurt those today because it reminds them of the discrimination they faced. This is the same as those with special needs. People with special needs are just that: People who have special needs. These people used to be treated horribly as if they were not human when that term was actively used.
4. Because you aren't just meaning, "stupid."
When you say, "That's retarded," you say it because people with special needs used to be called "mentally retarded." You're associating people with special needs as being stupid. They are not. They are competent and beautiful and deserve respect.
5. Because it has been taken out of the medical dictionary.
Saying someone with Down syndrome, autism, etc. is "mentally retarded," is also not okay. Neither is, "handicapped." Say the name of their specific disability or simply say, "special needs." Because that term is long gone.
6. Because they never did anything to you.
These people are the happiest, most loving individuals you will ever come in contact with. They would probably love you despite the fact you are being insensitive with your word choice.
7. Because I said so.
If I hear that word I promise it will not be ignored and it will not be pushed under the rug. It will be awkward and confrontational. Make it easier for both of us and be respectful to my friends and do not use this ugly word towards such amazing and inspiring people.