Can I get green card as an EB-5 dependent if the petition is filed one day before I turn 21?

Not impossible if you are not from a country subject to visa retrogression.

Probably, but you must also seek to acquire the visa within one year of it becoming available to you.

If the petition is filed and acknowledged by the USCIS before you turn 21, yes, you can get your green card. The law, the Child Status Protection Act (CSPA), allows children who turn 21 while EB-5 is pending to have their age frozen throughout the period. Once their I-526 petitions are approved, each child must either apply for subsequent adjustment of status or file DS-260 application for consular processing within two years. This opportunity favors largely children who are not from countries with retrogression.

Provided you not from a backlogged country, you can do it but you need to understand the seek to acquire provisions and take the necessary steps to freeze your age upon approval of the petition. It is doable but needs diligence.

The answer to your question is yes, assuming no priority date cut off or retrogression.

Provided there is no retrogression, yes, you should remain a dependent. If you later retrogress and have to wait for the green card, time spent waiting after I-526 approval will count against you, but the time spent waiting for I-526 approval will not.

It is possible.