You probably already know this, but I'll throw it out there anyway. Even a USCG approved inflatable life vest does NOT count as one...unless it's being worn.
Actually, that is not 100% correct.
The inflatable vests classified as a Type V pfd are required to be worn to count as a legal pfd. But, there are certain inflatables on the market that actually are classified as a Type 3 PFD for recreational use that don't require it to be worn to "count" as a pfd. Interestingly enough, if they are used for commercial use, they are classified as a Type V and they are required to be worn to be legal. I would guess that a guided fishing trip would trigger the commercial use.
The key to classification and use (must be worn, or not required to be worn) will be found on the USCG tag on the pfd. It will state if it has to be worn to "count".
For the original questions, if there is a legal pfd in the boat for the under 16 (meets the weight requirement), there is nothing wrong with the youngster wearing the inflatable. The exception to that would be if they were in a section of a lake/river below a dam where it is mandatory for all boaters to be wearing a legal pfd. Since the wearable is not legal for them under the age of 16, they would need to be wearing a pfd that is approved for them.