http://nationalacademies.org/hmd/reports/2017/health-effects...
The conclusions summary PDF has the highlights: http://nationalacademies.org/hmd/~/media/Files/Report%20File...
Personally, i first used weed when i was 22 and figure that's about right. I continue to use it in moderation. I would not be comfortable with its use by a minor child or a pregnant mother.