This might sound a bit weird, but Mari ia actually 33 years old at the time of the movies (2015). Hear me out.
In the manga (even though it is shit and might not be taken in to account when discussing the movies, but hey it's the only official info we have :/ ) it is said in the extra stage (the last chapter) that Makinami Mari is 16 years old, and an aquaintance of Yui Ikari and professor Fuyutsuki in the university of Kyoto. She is then sent in to Great Britain to study.
The year is 1998.
So that would make the year of her birth 1982.
So she is not the same age than the other children, she is closer to (actually older than) Kaji.
That is if we assume she has aged since -98, she looks pretty much the same in the manga than in the movies, in terms of body shape etc. etc. She is a bit calmer and not so eccentric, maybe that has come with the years?
It is also said in the manga that Mari is in love with Yui and very jelous of her skills.
Quite the turn, eh?
Well, it might be that the manga is set in an alternate reality, but as I said, it's the only data we have on her age :^)