I have 4 children and 1 due in just a few weeks. My 3rd child ( son) is 10 years old and has Down syndrome. He is AMAZING!!! Though we had prenatal tests done, all came out negative and we did not know he had DS until hours after he was born. He was my easiest of babies- he rarely fussed unless he was hungry and he smiled all of the time. He was born with a VSD ( hole in his heart) which did require surgery at 3 years old. He recovered beautifully. Yes people with DS do have mental and physical delays, but it does vary with each person. Some do have more health problems than others, but we have not experienced that aspect of it for ourselves.
Our son is very high functioning. He is in the 4th grade and spends 1/2 his school day with "normal peers" and 1/2 his day in a cluster class that focuses on his academics- reading, math, writing, etc. He does have speech/ language therapy in school 30 mins a day/ 5 days a week and occupational therapy in school 1x per week. He is very active and trains Brazilian Jiujitsu with the rest of the family. He is extremely loving and such a joy in our lives. For us, we have refused to treat our child any different than his siblings. He is expected to follow the rules and is disciplined when he does not. ( Of course we do have to evaluate each situation and make sure that the discipline he receives is appropriate for his understanding and knowledge of the issue.)
At times just as with any child, your patience will be tested but I assure you, your child will be the biggest blessing in your life!!!