Another alternative would have been "I don't think Will's mother wanted him to marry so young a girl", but that would sound too old-fashioned. Today we use the phrases suggested in #4.could we say like this : "but I don't think Will's mother wanted him to marry a so young (lady)"
I asked that because I also wanted to know how to say it in both situation. and I clearly know they are irrelevant. Thanks anyway!Maxscy, your original sentence tells us nothing about the age of the woman Will married. Will's mother is only worried about her son being too young to get married. So your follow-up query is pointless – you cannot assume anything about the age of the woman Will is to marry. Jektor has corrected the sentence you suggested, but it is irrelevant to the context.