Q:

How did wills work in times of Oscar Wilde?

Oscar Wilde's play The Importance of Being Earnest contains the following lines (shortened as marked for this purpose) in its third act:

JACK.  I beg your pardon for interrupting you, Lady Bracknell, but this engagement is quite out of the question.  I am Miss Cardew’s guardian, and she cannot marry without my consent until she comes of age.  That consent I absolutely decline to give.
[...]
LADY BRACKNELL.  [To Cecily.]  Come here, sweet child.  [Cecily goes over.]  How old are you, dear?
CECILY.  Well, I am really only eighteen, but I always admit to twenty when I go to evening parties.
LADY BRACKNELL.   [...]  Eighteen, but admitting to twenty at evening parties.  Well, it will not be very long before you are of age and free from the restraints of tutelage.  So I don’t think your guardian’s consent is, after all, a matter of any importance.
JACK.  Pray excuse me, Lady Bracknell, for interrupting you again, but it is only fair to tell you that according to the terms of her grandfather’s will Miss Cardew does not come legally of age till she is thirty-five.
[...]
CECILY.  Algy, could you wait for me till I was thirty-five?

From what I've found, the legal age (for self-decided marriage) at that time was 21 (and this is actually indirectly referenced by Bracknell). I could understand if the inheritance of the grandfather was only made available by the age of 35 (but that wouldn't really make any sense, as it would pose a problem unsolvable by Jack's consent), but was it actually possible to alter the legal age of marriage in such a way? Or is it just artistic licence on Wilde's part?

A:

For our purposes, "coming of age" does not refer to legal majority (aged 21), but the ability to inherit free and clear, through the will (age 35).
"Guardian" may be the wrong word for Cecily after age 21, but under the terms of the will, Jack was the custodian of Cecily's assets until she turned 35. Meaning that he could withhold her money until then if she married the "wrong" man. Such terms were unusually strict even for their time; most such wills would specify age 25, or age 30, at the latest. 
So the issue is not that Cecily needed Jack's consent to marry (after age 21), but that her inheritance would be delayed if she married against his wishes, which, presumably, would be reflective of her grandfather's. This was probably a device to force Cecily to marry the kind of man he (and Jack) favored. Only then would she get her inheritance before 35. If she waited till then, she would be an "old maid" and her marriage would be a moot point.

