"ret*d" in the mainstream is generally just a word used to express frustration or remark that someone wasn't particularly "smooth" about finishing a task--they did something inept, or else it's used to describe an idea that, in hindsight, was pretty bad. Some people are really mean, but I'd say the OP's customer may just have been frustrated and a little self-absorbed to think he could express his frustration that way, whereas the young men are probably just dealing with the insecurities of youth and a peer group that commonly uses that word. (I grew up with teens who used the word "gay" in such negative contexts instead--it's not meant to literally imply homosexuals, and "ret*d" isn't meant as a literal slur on people with disabilities--yeah, I know it doesn't feel that way, but most people who use such words in such ways have never even thought of what they might mean to someone who could be seen as falling under a more literal definition of the word, and if pointed out to them, they'd feel disconcerted and uncomfortable but would have trouble changing such a well-ingrained pattern of speech. Not that it's right, but it is a little more understandable.)
SeizeTheDay... I'm really sorry you have to put up with people treating you badly. I hope you feel better and don't get any further comments like this. ^_^ I think I have sensory issues so I have a weird gait and, like Emor, I've been made fun of for running, and also for other random tasks, and it's really damaging. If the feeling of being unable to present in a socially-lauded manner were experienced by these other people, they'd probably feel very sorry for having made fun of others.