The only correct response to your statement is "oh, I'm sorry".
This ^
School gate Mum was incredibly rude and insensitive.
Even if you had lost them when you were 40, if your perception was that it was young to have lost both parents (which it would be), then the only possible response to that is to say how sorry you are and how difficult it must be.
I was actually going to say, IMO, it is more of a "Stage of life" thing. So 'young' would be before they had the chance to be at your wedding, or to meet your dc or whatever.
19 and 23 is incredibly young and I'm really sorry for your loss 
My dc are 24, 22, and 19, and, although independent adults in some ways, are still at a stage of life where they look to us for support and advice. It would be incredibly difficult for any of them to come to terms with losing one of us, let alone both.
If it were U18, then I'd have expected the person to say "when I was still a child" or "in my teens" or something.