On the margin, getting old makes you worth less

I have been trying to think of good, practical examples of marginal analysis that I can use with my students (unlike many economists I know, I don't think it makes any sense to explain to students that 'if the movie stinks, you should get up and leave' - it might be the utility-maximizing thing to do but it's just too big a jump from human nature to be useful for teaching). EconomistMom may have a more useful example - she points out that when the marginal benefits from something (in this case, straightening out one's teeth) accrue over a lifetime, those benefits are automatically higher for a kid simply because they have more lifetime left. Economistmom goes on to talk about the implications for social policy (e.g., spending money on education versus cutting inheritance taxes); this could get into some nuances about investment vs. consumption but I think her point is a powerful one.