why is the extrapolation of data points from models beyond the time frame of the original data considered suspect?

Answer 1

e.g.
consider a graph a mother made of her child
the graph would continue to rise to about an age of appr. 21, then it would stay level, (actually starts to drop as you get older, as I well experience, lol)

If you were to look at the graph to the end of age 14 for example and saw no more of the graph, it would be totally false to assume it continue at that rate for, let's say age 24.