[ben_pfaff]

These days, it is absolutely unheard of for someone to get a doctorate at age 21. I don't know whether it was unusual then.

[rerx]

Stephen Wolfram got it at age 20, in theoretical physics, from CalTech.

(OK, that's also 40+ years ago by now)

[nbernard]

I'm not sure about unusual or not, but I believe it was (then and there) possible to start a doctorate right after graduating from high-school (if I remember correctly from a biography of J. von Neumann).

[f1shy]

I know plenty “Industrie Dr.” 22 or 23 yo