I wasn't actually involved in the amendment, but in the implementation. The Colorado initiative was titled an initiative to regulate marijuana like alcohol and very explicitly it made 21 the age, because that was the age for alcohol. As I said, there's a tension between that and our medical laws, which permit you to get a doctor's recommendation as young as 18. Around and near college campuses, that can be an issue.
On September 12th, 2017. See this statement in context.