Colorado law requires a 3rd and a flag ... Although there is not an "age"
requirement, the rangers seem to think they can interpret this as though there
is (and I guess, I can see the point):
No person shall operate a vessel on any waters of this state for towing any person on water skies, aquaplanes, surfboards, inner-tubes or any similar devices, unless there is in such vessel a person, in addition to the operator, in a position to observe the progress of the person being towed, and capable of relaying messages to the operator.
I guess the rangers get to decide whether they think someone is capable.
My 5 year old grand daughter is more observant and focused on working that flag and talking to the driver than most adults I know ... but she is not considered "capable", evidently.