I have two ancestors, "Henry" and "Lydia" who were born in 1837 and 1852. Henry and Lydia made things confusing for us, because Henry "forgot" his age a couple of times, and reported his year of birth as 1840 (and later). After years of research, I finally found out from another relative that he wanted to appear closer in age to Lydia, so "forgot" his year of birth during census years.

So, don't rule out that maybe Dora was changing her year of birth to be closer to George. Or, maybe she simply didn't know the exact year and was giving what she thought to be her age.
Update: I just reread your thread. Perhaps Dora
was born around 1842 and was giving her correct age on the census returns. It looks like it might be more likely that Dora (born 1834) was travelling with her Simmons family in 1853, and not with her husband, George Simmons. Thank you, Sandra, for providing the passenger list details.