[Norman99 (OP)]

Reproductively speaking, would it have been better for a man in caveman times to acquire a 12yo or a 20yo as a wife?

[syhprum]

I don't think in "cave man times" men thought much about wives and took any opportunity to reproduce that was available.
A twenty year old today would be considered more capable of producing and rearing live offspring but I do not think the difference is very large 

[alancalverd]

All depends on genetics and nutrition. Recent Europeans have experienced earlier menarche than their forebears, and this can only be attributed to improved nutrition, but we know very little about the soft anatomy and physiology of earlier species or even prehistoric homo sapiens. Skeletal remains suggest that pregnancy before 16 or so was unusual, and it is likely that teenage girls were more useful as workers than mothers. 

Our nearest living relatives, chimpanzees, gorillas and orangutans, generally tend not to reproduce below the age of 12 - 15 but menarche occurs earlier in captivity so again it seems that nutrition plays a key part.

Given the sedentary nature of urban life, it is quite probable that the optimum reproductive age for most modern women  is around 16 - 18, when a regime of school meals and exercise has had its effect and before alcohol, tobacco, fad diets and computers have begun to take their toll.