1/54 is still pretty long odds.
As for "the later years" let me tell you about my big (I'm 29 and she's 36, but she's definitely smaller than me!) sister, Blythe.
She has been living independently in her own apartment in a "normal" apartment complex since she was 21.
She plays the violin (can do all of Suzuki book 1) and sings in a chorus.
She works full time for a non-profit doing receptionist work.
She owns a Volvo and has had a drivers license since she was 26... perfect driving record.
She drives 3-4 miles, then takes the subway in to downtown Boston for work every week day.
She does her own shopping, cooking, pays her bills, etc.
She has a computer and uses it for email, checking her bank account, etc.
She has a more active social life than me.
She live about 20 miles from me here in MA, and our parents live in Utah... no problem.
The bottom line is that even in "the later years" someone with DS can live a relatively normal, productive life. It is by no means a "life sentence" for the person with DS or their family. That's not to say the challenges aren't huge... they are.
This might seem like an odd analogy, but I would compare the impact of having someone in my family with DS in their "later years" to having an elderly relative... you feel the need to check up on them and there are occasional logistical issues, but that person is essentially independent.
All that being said, don't start worrying too hard about something with 1 in 54 odds. No matter who your kid ends up being, your going to be their dad.
Here is a gratuitous pic of me and my sister.
