Margaret's birth year is in question:

British census, 1841 has her at age 30 (approx. birth year 1811)
British census, 1851 has her at ahe 41 (1810)
Burial register, 1857 has her at age 41 (1816)
Her tombstone states she died in 1851 at age 38, according to a person in Liverpool who transcribed the inscription.

The 1857 burial register birth date of 1816 would have made Margaret fourteen when she married. It is then most likely that the suggested census dates of ca. 1810 are most accurate.