if there is truly no difference in productivity from excellent programmers over 40 and "young" programmers why wouldn't there be companies that take advantage o

if there is truly no difference in productivity from excellent programmers over 40 and "young" programmers why wouldn't there be companies that take advantage of this obvious bias and hire primarily older programmers? Wouldnt the decrease in market demand for older developers drive their salary levels down and thus provide a large market advantage to the company hiring them exclusively? (obviously assuming equal or better creative and productive output) I know this is a classic laissez faire market argument. Hypothesis: On the whole, older developers are not as productive.