Hello, I have a question about basic first aid training for under 18 year olds.

We live rural, and my 16 and 14 year old boys recently went out with a bunch of local folks on a search in a forest / mountain for a missing 19 year old.  In another post, it was recommended that they get some training in basic first aid.  
We live on a farm, and with the family spending a fair amount of time using equipment and outdoors it does seem like a good idea anyways.
While they were searching, there were others with them that were trained in first aid , but any skill they can learn would be a benefit throughout their life - and they may not always be with others.

Where would I start looking to find training for people of that age?

Boy scout first aid handbook is simple.

Probably have local First Responder or EMT classes locally.

Try Stop The Bleed online as well.

Basic first aid is pretty easy.  Air goes in and out, blood goes round and round.  Work to keep that happening.

FM 4-25.11 First Aid: Army First Aid Field Manual: US Army

Written so someone with a 8th grade education can understand it.

Thank you for the advice.

BLS CPR

Stop the Bleed

Does you local Fire District or Law Enforcement have an Explorer program?   Typically they teach First Aid.

Call your local Red Cross.  At their age they could do a combo. class with CPR, First Aid and Stop the Bleed in a single day.

Check your local gun clubs for stop the bleed classes.  The clubs I belong to offer those classes.

My Son became an EMR at 14. That is an NREMT Emergency Medical Responder. It covers many things. Like if you arrive on an accident scene (mva, trauma, etc), what should I do?  It also covers the AHA CPR.  I dont think anyone really uses Red Cross anymore.

Now he is an EMT at 18.

Its a good course if you have a local fire department who puts one on.  Many times free, sometimes they cost. Its pretty minimal. Also stop the bleed classes are free online (no certification if that matters) I also took all of my kids when they were old enough to a neighboring fire district that put on safe sitter classes that gave them some more safety ideas when watching siblings.

There are also dedicated "Wilderness First Aid" courses. Our local college has one open each spring and fall.

Stop the Bleed and CPR can typically found for free at some of the above resources.

As others have said
Contact your local EMS or Red Cross
For a basic first aid/cpr class

EMT classes are more involved
But some rural jurisdictions will pay
for the class if you will work for a
year afterwards

There is a LOT of studying involved

Red cross classes are taught to the lowest common denominator buit can still be helpful.  I would opt for a wilderness course if available.  EMR would be great if you think he's got an interest.

I took EMT at a local community college when I was  17

some states still allow 16 year-olds to become EMT's this appears to be decreasing though.

This.

It’s probably more “work” than they want but many EMT classes can be completed in ~12 weeks. I did it when I was still in high school. Pennsylvania allows 16 year olds to become certified EMTs at the basic level.