You do gain some generalizable skills over time (and there are always exceptions to the rule) but in practice, older developers are more expensive versions of that which can be bought at a college career fair -- and the marginal difference in efficiency is offset by the young turks' propensity to work long hours while being paid in snacks.
I don't know if this is true. Even just with my meager 6-7 years of professional experience I can do things now that would've been impossible for me when I was starting out as a professional. Experienced devs aren't just more efficient - everything else being equal, they're more capable along all the axes you can measure a software engineer on.
Additionally if an experienced engineer has non-sucky people skills they can even turn your junior engineers into seniors with mentoring and example-setting. Which means you'll be employing a senior engineer for the price of a junior engineer, at least until they learn their increased value and ask for a raise/move on. /s