Is the age of EB-5 dependent children still frozen after the approval of I-526 and before a visa interview?

That is correct. His age is "frozen" while the EB-5 petition is pending. Unless you are a national of a backlogged country, he may be able to process as a dependent if you are able to quickly file the immigrant visa application upon the approval of the I-526 petition.

Provided your country of chargeability is not backlogged at the time of approval, and you take steps to "seek to acquire" status by filing the DS-260 and paying the fee bill, the age of the child is likely protected or frozen.

You will also have up to two years after the I-526 is approved before he will be considered that he has aged out.

No, the age is frozen only when petition or application is pending; the risk is that your son could age out if there are no visa numbers available at the time the petition is approved.

It depends on whether EB-5 is backlogged from your country of birth. If it is not backlogged, since you filed, now or after, the age is frozen.

If you as the investor is a citizen of a country that is not retrogressing, then your child who was under 21 years of age at the time of filing the I-526 petition, and now over 21, should still be able to apply for the conditional permanent residency visa.

It all depends on when his priority date becomes current and his age then. He must file if under age within a year of the priority date being current.

Unless you are from a country facing heavy retrogression, that will probably be OK, but it would be a great idea to pursue the green card as quickly as possible without delay.