My own children were baptized as youngsters and not infants because my husband and I hadn't found a church until then.
But their baptism was not predicated on their consent to become part of God's family, but on the recognition by us and our congregation of the gift of membership into God's family which God bestows on His own from before the foundation of the world.
It's kind of like the growing awareness of predestination. At first we say we're on our own. Then gradually we give God some credit and keep most of it for ourselves. Little by little we become aware that it is ALL of God according to His plan for His creation. And the totality of that awareness is life-changing and amazingly comforting.
So, too, IMO is the understanding that our baptism is something we do in time which illustrates and affirms what God has already done for us from before time by the "Lamb slain from the foundation of the world."
Amen
Roms.9:11 (for the children not yet being born, nor having done any good or evil, that the purpose of God according to election might stand, not of works but of Him who calls),
Little by little we become aware that it is ALL of God according to His plan for His creation.
So why say Baptism is sacramental and imparts Grace? It's no different than saying the Lord's Supper is sacramental and imparts Grace.
We don’t baptize infants but we do have a dedication service where parents and the congregation agree to help raise them in the nurture and admonition of the Lord. We take that very seriously. Later, when they are old enough to receive Christ into their lives, they are immersed in a lake nearby. It’s meaningful to them and to those of us who are there.