A 16-year old should be fine to be a primary applicant. With regards to the aging out issue, the moment a petition is filed, the age of the designated child will be frozen until the petition is approved under the law, the CSPA. Consult an EB-5 attorney for appropriate planning.
If the project documents are done correctly under UTMA, a 16-year-old can be the principal. Again, the documents must be done correctly, and if your son re-confirms the subscription agreement at 18, you should be fine.
A 16-year-old may qualify to be a principal EB-5 investor if properly structured under applicable U.S. state law. Hire an experienced EB-5 attorney to consult you and competently prepare I-526 petition.
There is no minimum age requirement. Most regional centers require the minimum age of 18 for the child. However, if the guardian of the child can sign the paper for capacity purposes, then it should be OK. Also, please note that as long as your child's I-526 petition is filed with the USCIS prior to your child turning 21, then there would be no impact or age-out issue.
As a person born in Vietnam, if you were to apply today as the main investor with your 16-year-old child as the dependent, it is not clear to me that s/he will age out for sure. You are correct, though, that you would be taking some risk. As you must know, when you apply for the EB-5 and file the I-526 petition, your child's age is frozen. Depending on how long it will take for your I-526 petition to be approved, your child may or may not age out. Once your I-526 petition is approved, your child's age would be unfrozen at that moment. Due to your child's current age of 16, if your priority date does not become current within roughly five years of your I-526 approval, your child could age out. That said, there are no statutory age requirements for filing the EB-5. However, there are other contract law considerations on a state-by-state basis. As a practical matter, as the sole investor, it might be difficult for your child to be compliant with all the requirements unless s/he is at least 18 years old. I assume you will gift the required funds to your child. You also need to consider security laws that deal with accreditation issues. Finally, if you have decided that your child should apply on his/her own you might want to wait until s/he is 18 to be on the safe side.
Yes, a 16-year old can file an EB-5 petition on Form I-526. Anyone who files individually will not age out. The question is whether or not if you file at this point and your child ages out. That is certainly a possibility. If money is no object, you can file for your family, and your son independently can file his own EB-5 petition to be safe.