It really depends entirely on the condition of the lens. I have numerous Schneider Compon's and Componon S' some dating from the late 60's and early 70's and they are just as good as a modern new lens in practice.
However enlarger lenses aren't Multi coated (which hardens the glass surface) and they are more prone to cleaning marks and I have seen degradation in prints due to very slight flare cause by almost invisible cleaning marks. This is as likely to happen with a modern lens as a 60's/70's if badly treated.
So you need to look at a lens very closely before buying.