John Lightfoot on Luke 1:36

[Hath also conceived a son in her old age.] The angel teaches to what purpose it was that women, either barren before or considerably stricken in years, should be enabled to conceive and bring forth; viz. to make way for the easier belief of the conception of a virgin. If they, either beside or beyond nature, conceive a child, this may be some ground of belief that a virgin, contrary to nature, may do so too. So Abraham by faith saw Christ's day, as born of a pure virgin, in the birth of his own son Isaac of his old and barren wife Sarah.