Word for top and bottom of range [duplicate]

In mathematics, extremum refers to the minimum or maximum values of a function, while endpoint refers to the start and end of intervals.

These can be referred to as edge cases or limits, although these terms are not usually used to refer to children in a family (but work fine for numbers).

In general if you're talking about the most and the least of something, they can be called the "extremes". (Or the "extrema" in technical mathematical context, as Jasper notes.) Like you could say, "The extremes for the stock market average for this month were 11,182 and 12,247" (made-up numbers -- I didn't check actuals for this example).

But it would be unusual to use this, or any other suggestion I've seen on this thread, to discuss oldest and youngest children. I really don't know of any word or phrase for that other than "the oldest and the youngest".