Disney isn't the most original company on earth, especially when it comes to story telling and are pretty lazy
Disney Film Sources
At the above link one can peruse the fairy tales that Disney has used for their films, a few of them being:
Film - Source - Author, Year of Death
Snow White - Snow White, Brothers Grimm, German, died in 1859/1863
Cinderella - Cinderella, Charles Perrault, French, died in 1703
Aladdin - Arabian Nights (Antoine Galland's version), French died in 1715
Mulan - Hua Mulan, a traditional Chinese Story
The Little Mermaid - The Little Mermaid Hans Christian Andersen, Danish, died in 1875
Beauty and the Beast - Jeanne-Marie Beaumont, French, died in 1780
But to be honest, it's all fine. Because stories are constantly being copied and modified or just stolen from everyone since the beginning of telling stories...the modern film scene isn't exactly riff with original stories lately...
To get to your question,
Why did Disney do this in their stories? Because well, the stories they took from did, and Why change a perfectly fine story for children who don't know or even care about the multi-cultural and Western societal hookups and taboos of fantasy marriages?
Your real question should be, Why do the authors of the stories write stories with such young marriage ages.
The two things in common with the examples I listed at least, is that none of them are from North America and all of those stories are a couple hundred years old, and two of them even older AND from non-western societies.
So it's hard to put them in a North American perspective and criticize it for not holding North American marriage standards 3 hundred years in the future. (Well, we can and should, but that's a different discussion)
So, if we look at typical age of marriage at the time in some excerpts:
...in most of Northwestern Europe, marriage at very early ages was rare. One thousand marriage certificates from 1619 to 1660 in the Archdiocese of Canterbury show that only one bride was 13 years old, four were 15, twelve were 16, and seventeen were 17 years old; while the other 966 brides were at least 19 years old
In France, until the French Revolution, the marriageable age was 12 years for girls and 14 for boys. Revolutionary legislation in 1792 increased the age to 13 years for girls and 15 for boys. Under the Napoleonic Code in 1804, the marriageable age was set at 15 years for girls and 18 for boys.
And just to put this into an interesting perspective,
...No person who is under the age of 16 years may contract marriage.4 These provisions were enacted in 2015. Before 2015, it was possible for children less than 16 years old to get married in some jurisdictions of Canada....
3 of the 6 authors of those stories in the small list I gave are French, and apparently in Canada until before 2015, you could marry at any age it seems as long as you had parental consent!
So to answer your question, Why is there such a disparity in age for marriages, in Disney films?
It was common at the time, is apparently still common, and Disney is a lazy Corporation that cares about money and not what children think, or their parents.