In 1754, the Parliament of the United Kingdom passed Lord Hardwicke's Marriage Act. The act, however, did not cover the United Kingdom; it merely covered England and Wales. A parent could veto the marriage of any child under the age of 21.
Now, for most of the residents of England and Wales, this rule was absolute. Travel to Scotland was only slightly more thinkable than travel to the moon. To those with whom Regency Romances deal -- the aristocracy and the upper reaches of the gentry -- travel to Scotland was possible, and elopement to Scotland was one option that young couples thwarted of marriage would, at least, consider.
A road was constructed in 1770 which was the fastest land route from much of England to Scotland. Gretna Green was the first village in Scotland on that road.
Gretna Green became synonymous with elopement. Couples could elope to many places -- from London, a trip by sea would probably have been faster in those days -- but you only threatened to elope to Gretna Green.
While the act applied equally to sons and daughters in wording, among the people who counted at The Season, few men were married before age 21, and most women were.
A girl was typically 16 when she took "her Season." While The Ton attended balls for every Season thereafter, the next stratum of society down did not. A girl who turned 18 unwed was a reject, and she had to choose between spinsterhood and a less suitable marriage than she had previously considered.
In such a social milieu, waiting until you are 21 was an intolerable torture.