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? My ideal career is to be lead a lab at a university as a professor. 

If I completed a residency and fellowship after an MD-PhD, which I think I'd love because of the 80/20 split that fits my interests, I'd be in my mid to late 30s by the time I was ready to start an independent career, as opposed to a Ph.D., which I'd complete, with fellowship, around age 30. 

I've heard the adage about scientists who don't contribute their biggest success before 30, and I'm concerned that close to 40 is far too late to start as an assistant professor. 

Would I be better off going into a Ph.D.?
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.