It is driven by the insurance companies. TechShop has had various policies over the years I've been a member, all driven by the insurance provider. The day the insurance got turned on for TechShop #1, my daughter, (then 7 or 8 or so) and I went in to help demo drywall and so forth to start the shop build out. Later, the minimum age became machine-by-machine, and always with a parent/guardian present. Laser cutters and sewing/surging machines had a minimum age of 12 at one point, and at the same time the Bridgeports had a minimum age of 16. Not sure about other machines. Not sure what it is now -- it tends to change as the insurance carrier thinking evolves -- and insurance is, as you would imagine, a big line item on the expense side of the P&L.
So, remember, we are talking insurance company thinking here, so normal common-sense thinking does not apply. The thinking is driven by statistical tables, recent legal settlement amounts, and the personal gut-check of the lead underwriter's visceral fears. Given all that, I think TechShop has had a reasonable experience with insurance, despite my daughter not being able to get at many of the machines for age reasons.
Unfortuantely, there is no "brain check" that can work -- I used to each one of the safety-and-basic-usage classes (SBU's) for one of the machines. It is designed to make you safe to use the machine, and in my experience age has little to do with how safe a person is. There was one 50 year old mechanical engineer who had obviously been a paper-pushing engineer for 25 years, because during one incident the main thought going through my mind was: "Good God, man, can't you *hear* that is a *very* unhappy machine???", while other people with much less background were safer to be around.