https://www.bizjournals.com/losangeles/news/2017/04/03/snapc...
The above shows the split of users over 18, but isn't helpful is showing how many are age 13-17 and the proportions.
https://www.statista.com/statistics/326452/snapchat-age-grou...
According to this, 80% of users are over 18, with over 50% falling into that 18-34 block.