Pointless arguments

I know I might be faced with people who ask rude questions or say ignorant things. I've already had a couple and it's only been 5 months since we found out. I have a coworker who is probably my biggest annoyance at times with these things. He doesn't mean to be hurtful, he's almost just to stuck in his perfect little world to realize that some people don't have unrealistic lives. I don't mean to write specifically about him to hate on him or to get people to hate him. I want to share because it's real. He is not alone in the way he thinks. He's a very kind person and has actually been super nice to my son the times he's seen him.
The first time he met my son it was at a work dinner for a couple employees that were going away. I still hadn't known about my son's condition, but it was obvious my son was delayed. He didn't talk (still doesn't) and he scribbled and threw things like a baby. He was already two and my coworker who has nieces and nephews of around the same age asked why he didn't talk, why he made his lips the way he did, why he didn't really pay attention. I had been use to these kinds of questions because they were questions I asked myself, but I brushed it off and I told him staight up that my son was slower and it happens. Obviously my tone was a little more on the rude side, but I didn't have answers and that frustrated me more than the questions.
When I got my diagnosis, I was at work and only my boss knew about it, but he hinted to the other members of our team that I was having medical issues with my son and to give me some space. Of course eventually they asked if he was ok and if there was anything they could do. I think they imagined much worse, but I finally just told them my son had Down Syndrome, I felt it easier than explaining the percentage and wanted to avoid the how and why questions. For the most part they have been very considerate and whenever my son comes around they play with him and give him suckers. My coworker who is a teller with me is the one who is more outgoing, I guess you can say, with the comments he makes and questions he asks.
The most recent conversation we've had started when a customer of ours came in with his daughter. He was an older gentleman, probably in his late 40's or early 50's, and he told us his daughter was 6. My coworker laughed and joked on how she could be his granddaughter, something I'm sure he's heard a lot before because he just smirked and tried to hurry with the transaction. After the customer left, my coworker said, "I wonder why that little girl wasn't born with Down Syndrome, because that guy was pretty old so his wife must've been older too when they had her." I was kind of stunned by his comment. I asked him why he would wonder that and he said because he's heard that the risk of a child being born with Down Syndrome is higher for older women. I told him my mom had my sister in her late 30's and my sister was fine. He said either way he wouldn't want to take that risk. I asked what risk would he be taking. He said he wouldn't want his wife to get pregnant at an older age because he wouldn't be able to handle a child who would never be able to take care of themselves. He wouldn't want a child with Down Syndrome because it would be too much work and it's like having a child for the rest of your life, according to him. I told him it doesn't matter what age you are if you're meant to have a child with a disability it will happen. I told him I was 19 when I had my son and according to tests I had almost no chance of having a child with a disability, which I did. He started going on about how either way he wouldn't want that on his shoulders. I was just shocked at how dumb he is on the subject. How much ignorance a person could expose in less than 15 minutes. I'm sure he is not alone in his theories and it's sad to think people are this naive about myths. I know I wasn't fully educated on raising a child with a disability, but I also wasn't so against it. I wasn't always concerned about having a child who would be a little extra work. I was always willing to accept my child with a disability, which is what I had told my parents when I took that test when I was pregnant. Yes, it was a relief at the time that I had gotten a slim chance, but even now it doesn't change what I would do for my son. It's actually kind of funny to hear some of the things people, like my coworker, have to say. Sometimes it'll hurt my feelings and I'll be rude about people's comments, but I'm starting to just let it go. It's easier to sit back and smile because at the end of the day, my son is happy with what he's got.