@Nick ******
I gave you the only answer there is. There is no rule for Muay Thai or Thai Language. The ministry of education, the MFA, and the immigration department do not have a rule that says there is any age limit. But that doesn't mean an immigration office or embassy doesn't set their own rule. So you want us to what? Call every immigration office in all 77 provinces of Thailand and tell you which ones have an age limit? You have to go to your immigration office and ask. But any school in that province would know the rules since the schools are the ones who send people to file for the visas.