I'm running S-PLUS 3.4 under Unix, under SunOS 5.3. So it appears that there is a very strong nonlinear effect of age. Rugplots show that the data are fairly dense in the interesting places.
Residual plots seem fairly ok.
I omitted three negative outliers. The gam model has surely over-fitted. I guess the lesson is that one needs to use every available clue to what may be happening. I wonder what insights contributors may have. Incidentally, StatSci really does need to do something about the 299.478758987789 degrees of freedom.