Some 16 year olds are more mature and experienced with firearms and equipment than some adults. Dad taught my brother and I to be responsible with firearms and everything else. He taught us to evaluate the situation and do what was the safe. Experience - started driving farm equipment with dad when I was 6 - worked with him and beside him for years. Started shooting with dad about 7. Hunted and trapped with my dad and older brother. There was more than one occasion growing up when I was in a firearm store and a customer asked if I could assist them (thought I worked there).
Maturity, experience, attitude and knowledge should be the major factors - not age. Age is not always an indicator of maturity or experience. I will take any responsible 16 year old the father trusts to be around heavy equipment and firearms to a 21 year old who has little to no experience as long as the laws are being followed. Like I said, we worked, shot and hunted with and beside dad for years. He did not worry about use doing something stupid. He was concerned about some other adults and their kids doing something stupid.