Well it’s not like Catholics, the SBC only baptizes adults, and they have to be “true believers” and profess a “personal relationship with Jesus Christ” and all that before they’re allowed to be baptized, so I don’t see how assume less baptisms mean less members? What do they consider households where they have 8 kids under 18 coming with them to church, are the kids not considered “members” of the church? Confusing.
The SBC doesn’t just baptize adults, they baptize children and teens but they try to make sure that the children involved have reached an understanding of what baptism and their personal salvation is all about. Many SBC pastors will speak of “reaching the age of accountability” when a children starts to understand right from wrong and can personally distinguish the pull of conscience....yay or nay. The ancient Hebrews (and Jews after the first diaspora) established that “age” as being around 12 or 13 years old.
No, only those who have gone through Believers Baptism are considered members of the church. That is not limited to adults.
I was a member of a SBC-affiliated Church for a while so let me see if I can help you get a better understanding of what's going on.
First, while the SBC does not practice infant baptism, that does not mean that they will not baptize a child who is over the age of accountability and who has made a profession of faith.
If anyone comes to a SBC Church and makes a public profession of faith that Christ died to atone for their sins, then they will be baptized. The pastor and elders of the Church will speak with the person, to try to ensure that what they are doing is a genuine profession of faith and not just an emotional response to the Gospel. There has to be an understanding that this isn't something that is based in emotion, even though emotions are going to be present.
Furthermore, baptism and Church membership is not the same thing within the SBC. You can be baptized within a SBC Church and become a member of another denomination. Baptism grants you membership in "The Church", aka the Bride of Christ, but membership in a local body of believers isn't automatic or even required by the SBC.
Finally, membership in the local SBC and attendance at that same Church are measured separately. Every SBC Church, as well as most other Baptist, Pentecostal and Independent Church that I've ever attended had a plaque somewhere that listed things like members on the roll, members attending, total attendance, offerings and other Church business data. That family with 8 kids under 18 which you mentioned above would not be counted as members if and until they asked to be accepted into the local Church and were accepted by the Church. Until then, they would be counted as attending but not members.