In choosing between MD-PhD and PhD, how should I factor in the expected age at which I'll start my career as a professor?
According to the NSF's report, the median age of a PhD recipient is 31.6 years old. That means you'll only be a little older than a typical graduate.

This age shouldn't be shocking considering in the US it often takes 10 or more years of college to obtain a PhD, so that means that most people will be late 20s or early 30s if they never went to industry or didn't graduate early.