From "The Oxford Guide to Family History" by David Hey.
Another myth that has been firmly laid to rest by social historians is that English people used to marry when they were very young......The evidence is inconclusive for the medieval period, but the study of parish registers has left no doubt that since the sixteenth century at least England has shared the north-western European pattern of a later age at marriage.... So well established was this pattern by the beginnings of parish registration, that it is unlikely that medieval experience was much different.... Only a very small proportion of the population of Elizabethan England married before the age of 20.
Stan