What defines the boundary of the *observable* universe concerning the age limit factor?

Answer

Light from objects farther away than about 13.8 billion light-years has not reached Earth yet

Because the universe began approximately 13.8 billion years ago, light from objects beyond that distance has not had sufficient time across intervening space to reach our vantage point.

What defines the boundary of the *observable* universe concerning the age limit factor?

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Why is it Dark at Night?

Earthcausenightdarkrotation