When Adam sinned, we were in his loins, so to speak. So the sentence of death included us.
فَقَدْ سَبَقَ وَحَذَّرَ آدَمَ أَنَّ ٱلْأَكْلَ مِنَ ٱلثَّمَرَةِ ٱلْمُحَرَّمَةِ سَيَؤُولُ إِلَى مَوْتِهِ —‏ وَهذَا مَا حَصَلَ فِعْلًا .‏