I've been searching over a lot of websites and their are some online HVACR schools that needs 18 years old. But then when I dig much further, it seems that Maine contractors doesn't need license in Maine.
source:http://www.nationalcontractors.com/hvac.htm
http://www.nationalcontractors.com/hvac/mehvac.htm
Although a blog post says that the state requires them for an EPA certification.
source:http://www.hvacjobstraining.com/maine-hvac-license/