Your spouse and unmarried children under twenty-one may be included as derivatives. Whether a child near twenty-one keeps that eligibility depends on age-protection rules tied to the petition’s filing and pending time, so his case needs individual advice before the filing date is chosen.
The son’s birthday is a filing-date question, not a hope
Derivative eligibility is fixed by relationship and age: the spouse at the time of the petition, and each child who is unmarried and under twenty-one. A child who turns twenty-one during the process may be protected by the Child Status Protection Act, which can subtract certain pending periods from his age, but the calculation depends on when the petition is filed, how long it is pending and when a visa becomes available. Do not assume the outcome; have counsel run the calculation against the intended filing date, because moving the filing forward by a few months can matter.
Each derivative receives conditional residence with the investor and is included in the Form I-829 removal of conditions. Derivatives are not required to invest or to participate in management. If the son will not be protected, his alternatives are his own future category or a later family petition, both of which take time.
Married children are not derivatives at any age.