I think it depends on the kid, where their money management skills may be lacking, and what you want to teach.
If the kid will learn and internalize a valuable lesson through the purchase (drive snowmobile it off the lot and immediately lose X% of the value), letting him make the purchase serves a purpose (and prevents costlier waste in the future).
If you think the kid needs a framework, while a minor 50/50 save/spend rule (or 70/30 or whatever makes sense; if kiddo gets a pile of cash for his birthdays you could allow the first $X as spending money). The lesson learned here may hit home later, when he can afford school without big loans, or to pay cash for a car, or make a house down payment.
Hopefully you are teaching financial lessons in everyday life, and not starting at 16. But seeing as people find MMM in adulthood, middle age, or older and can make changes, 16 is definitely not too old to learn.