Am I eligible for EB-5 if I get the I-797 notice after I turn 21?

Yes, you will still be eligible.

Most likely yes as your age is locked in while the I-526 is pending. There is a complicated law called the Child Status Protection Act.

Under the Child Status Protection Act (CSPA), your age at the time a visa number becomes available is subtracted from the amount of time the immigrant petition was pending - the filing date to approval date. If the resulting age is under 21, and if you seek to acquire lawful permanent residence status by filing for immigrant visa processing or adjustment of status within one year of being eligible to do so, then you are protected under CSPA and continue as a dependent.

If you are from a backlogged country, such as China or Vietnam, you may have a problem. If not, you have one year to seek to acquire a visa by filing form DS-260 and paying the fees, or by filing an adjustment under chart A final action date.

The date of filing is what matters, not the date on which the receipt arrives. Check the date of filing, that is your priority date. Remember you will have to file your green card application or immigrant visa application prior to turning 21 under CSPA.

The date you get a receipt does not determine your eligibility. The date USCIS receives the application determines if you were under 21.