The function is your height at age .

Horizontal line test:

A function is one-to-one if and only if no horizontal line intersects its graph more than once.

That is, any horizontal line in the graph must cross the curve at one point only.

The path of height is initially increases after certain age it will remain constant.

Graph: 

Graph the rough sketch of  height at age .

Draw the horizontal line .

Observe the graph:

The horizontal line touches the graph twice.

.

The function is not an one-to-one, because it violates the horizontal line test.

The function is not an one-to-one function.