Maria claims she is 17 at the beginning of the series. Between the first episode and the last episode over an entire year passes, Christmas happens twice and everyone ages a year.
Except for Maria of course, she remains 17, so so she claims.
There are only two possible explanations for this. The first is that she is lying, the second and more likely of the two is that she was born on February 29th on a leap year and that she is actually around 68 years old.
I will admit she looks really good for 68.