I bet the 20 yo is in for a surprise. If you take the pistol away from the attacker then there is no longer a lethal threat, no justification to shoot. However, if in the process of disarming, the pistol fired and bullet hit the bad guy then he might come out alright.
Objectively, the attacker could attempt to recover his lost gun and has already demonstrated a lethal threat in the initial attack. If the victim is already wounded from the previous attack, the threshold for "lethal threat" goes way down. Only an explicit surrender from the attacker establishes the end of a lethal threat. Or the attacker can back away, unarmed, to establish the end of a lethal threat.
What the attacker cannot do, is stand there bleeding and swearing at the victim or advance on the victim in any way. That does not end the original threat. The attacker must (1) Withdraw from the fight and (2) Effectively communicate that withdrawal to the other person.
This is a minimal the "Standard of Reasonableness" in all States. See "The Law of Self Defense" by Andrew Banca. pp 49 (Withdrawal/Communication).
But laws do not matter as much as they should. In practice the treatment of the case depends on the jurisdiction and the political views of the District Attorney. It also depends on the race of the attacker and victim. Under the current pretense of law, a black attacker may well have charges dropped if a white victim counterattacks. And the white victim may be charged with a crime for resisting.