These words need to be judged in the context they are used :-

So in the Legal context of BMD's etc :-

With parental consent they were able to marry from age of 14 and 12 , over 21 no parental consent required , under Hardwicks Marriage laws of 1753 (?)

Spinster in its general sense just means status of female unmarried- its opposite being batchelor. ( other recorded status would be widower or widow, and occasionally divorced)

In the context of a Novel /Film etc :-

Spinster seems to often imply or be used with, an older female !