Answer: One passenger under 21
The clock starts when the license is issued. For the first six months, carry no passenger under 21 unless a parent or another licensed adult driver is in the car. After six months and before one year, you may carry one.
Easy to get wrong: Using none from the first six months, or the seat-belt limit from after one year. Brothers, sisters, and medical emergencies do not count against that one.
Colorado Driver Handbook (DR 2337 (12/05/24), posted as the January 2025 handbook; the current edition on dmv.colorado.gov/drivereducation on 2026-10-30), Chapter 2, p.2