Average is a neat little math trick to use here. Median is a much more meaningful number to use

Imagine 5 people use TikTok with ages of: 10, 15, 15, 15, and 65

The average age for that group is 24. You can technically claim the average age of those users is an adult, out of college. The median, however, is only 15. 15 is a much more accurate descriptor of that group than 24. It's obvious why TikTok is choosing average here. It's very easy for a small segment of older users to drag the average way up