Both my daughters took it at 10, with my oldest taking her oryx that same year.
As mentioned by Elksniper ( a heckuva model for youth hunters in this state ), 10 is about right.
I found it helped that they had a STRONG grasp on reading. Though new mexico is really good about working with the students that are younger, even reading the test to those younger than 11.
As said by 1Fast and NMPaul, GET HIM IN THE FIELD as soon and as often as you can. Being exposed to firearms so they aren't a "Big SCARY Deal" but rather something he knows about, respects, and knows he can ask you about, rather than something that seems like a mystery, and he feels he has to "sneak" to see/touch.
Remember how much our parents saying "STAY AWAY FROM BEER!" worked with us?
The youth are our future, and you are an awesome dad bringing him into this lifetime passion of yours/ours.
NM is such a great state for youth and hunting you are in the right place!!
Look at the NM Hunter's Ed page for the online training ( a great place to test the waters on his comprehension/absorption of the subjects ), this is the first step in getting in the classes anyway, and then sign him up!
http://www.wildlife.state.nm.us/education/hunter_ed/index.htm
Know that they fill up fast!!! Today I logged in the young man that is mentoring with me, for the class Nov 12-13, 30 slots, and they had to be filled in something like 15 minutes. I am so thankful we got a seat ( we have been trying since May, but with school, sports, life... )
NMYB
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Teach your kids to hunt now, and you won't have to hunt for them later.