Is your question about the recipients of baptism or the mode? It seems you're most emphasizing that Namaan was an adult.
Namaan was an adult. The OT pattern is for adult professors/ conversions such as with Abraham (Romans 4:11) or Namaan, the reality comes before the rite/sign (and as a token of their faith which preceded); but for children of believers, the sign is actually commanded by God himself to be administered long before the reality is possible (Genesis 17). We argue this same pattern carries over but now the NC sign is baptism. For adult professing believers, the pattern is faith in the Lord, then baptism; for their children it is baptism, then faith in the Lord.