I remember this girl in my class in 7th grade, before summer vacation - end of 6th grade - she was like all the other girls in the chest department, barely anything there in terms of breasts, but then she comes to school at the start of 7th grade, and suddenly all the girls - and boys, obviously - surrounded her and wanted to talk to her, probe her with all kinds of questions and such.
When I finally saw her, I quickly realized the reason, she had grown breasts the size many adult women would've wished they had, and all the boys wanted to see them, probably touch them too, while the girls were varying between protecting her and being equally as curious as - if not more so than - the boys were about her breasts, difference being that they were all girls, and that kind of talk is - I'd assume - common between girls.
Point of the story being, I can understand how easy it is for a 16 year old to fall in love with someone like Ferris, she looked older than 13, especially given how blessed she is in the breast department at such a young age, and she was working in a restaurant that would normally require you to be 16 at the youngest I'd imagine, 18 normally. All things pointed towards Ferris being at least as old as he was, if not older as he said himself. This was simply a case of 'Love makes you blind.'