Art after dark

We went diwntown tonight after dinner and were at a store looking at kids art and a woman was studying Liam for a minute. Then she said, "he's very cute. I used to work with special ed". And it was an interesting moment. It made me think: was it necessary to even mention that? But then I said, "Oh thanks! Yeah, he has Downs.". And then I wondered, was it necessary to say that?
It's the first time a stranger in public has brought it up. Earlier today I was thinking about how we're starting Parent Participation next week and wondering if I will mention T21 from the get go so people aren't wondering. I was thinking of even saying that the other parents could feel free to ask me questions. I myself didn't know anyone with DS until Liam was born.
What would you do?