One is left to presume that Peter in the fulfillment of Joel includes "nursing infants" even if they were not physically present at the time of his sermon at Pentecost.
All tagged Joel
The Audience of the New Covenant in Joel (Part 1)
So, Joel’s present audience remains in some sense the future audience of Peter. In every case this future generation is the generation of Peter. Thus in his sermon, Peter is picking up Joel's prophecy.
The Holy Spirit's Covenant
Contrary to popular opinion, it isn't enough to just "read the Bible" and make a decision. One has to struggle to identify their own paradigms and lens. Everyone has them and it is always hard to break away from them (even if they are correct) to evaluate the Scriptures afresh.