At choir practice

We sat together in the alto section. She pointed to the music, measure 30.
"That's 30," she said.
"Yes," I said, "I see it. A three and a zero."
Suddenly we weren't talking about music.
She: Thirty. After that everything goes down, down, down.
Me: Oh. And how old are you?
She: Thirty.
Me: (Thinking that thirty is not the point in life after which everything goes down, down, down.) Oh, yes. I see. But really, it's not so old.
She: I hope I live to be your age, when you just don't care about anything anymore.
Me: (More than a little taken aback.) Oh, you can see that by looking at me? (Thinking I looked pretty good, thank you, and knowing I had taken a bath that morning.)
She: It's just that I was shocked when I heard how old you are because you still have really good-looking legs.
Me: (Half under my breath) That's not all.

Thank goodness we had to sing again, and this strange conversation was ended.

Obviously, I have reflected on the conversation a time or two. And I do not know what it means.

Like, what does she mean by "when you don't care about anything anymore"?
Why does she think her life is nearly over at age 30 and on the down, down, down path?
Does she not know that she has 40 years before she's my age and can do SO MUCH in 40 years? Forty years!
And why and how often is she looking at my legs?