It goes by height and weight. But there are a variety of rides for all age groups.
From what I found, there are different height requirements, but there don't seem to be age restrictions. The range are under 36" with chaperone, over 36" alone, under 42" with chaperone, over 42" alone, and over 48" alone. Granted my source before is 7 years old, but I'm sure it hasn't changed a ton. https://bestkidfriendlytravel.com/2012/06/25/mall-of-america-nickelodeon-universe-a-guide-to-the-rides/
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