I think it varies too. Like all people, people with DS are individuals and their experience in life is different for each person. I know a couple of my religious friends really want to believe DS kids and adults are always happy. It seems to be some sort of thing intwined with being pro-life.
I guess we would have to ask DS people if they feel happy to really know how they feel. I would assume they know they are “different.” I don’t think anyone likes feeling different, especially when they are children, but I am just assuming. Growing up I used to play with a DS boy. I didn’t even know he had DS, or that he was different, until my mother told me. Not that she was saying anything negative, she was just fine with me playing with him, and she was very friendly with the boys mom. I don’t think he was always happy. I remember him frustrated at times, and I think he had some health problems related to his genetic abnormalities. By the way, many DS infants are born with health problems. Some of the most serious are digestive and heart from what I understand, which can require surgery. Almost half of DS children are born with a heart defect. 15–20% have serious digestive problems that need surgery almost within days of birth or they would die.
Sort of related, but not quite what you are asking; my grocery store has a lot of DS employees. I think it probably helps DS people feel good, but I really can’t speak for them of course. Randy Lewis who had a very senior position at Walgreens implemented an initiative to hire disabled people, including DS and autistic people, at their distribution centers. It was wildly successful, and other companies have adopted his plan. He wrote a book if you are interested titled No Greatness Without Goodness.
There is a lot of work for the parents that’s for sure. Not to mention the worry parents have if they themseves died. All parents are terrified of leaving their children without a parent, but the fear for their DS kids usually last their entire lifetime.