Number one you are putting the cart before the horse.
Number two, we are NOT UNDER THE LAW.
Please stop using OT comparisons when we are dealing with NT realities .
There is a reason for the New Testament.
If Jesus wanted to set the example of infant baptism (not circumcision) He would of been baptized as an infant.
He wasn't
We are not under the Law, but the Law isn't something you're free to ignore. We know a lot about God due to the Law, and these things tell us how God interacts with His creation.
God's Covenant with humanity began with a couple (Adam and Eve) It extended to a large family (Noah) and grew to encompass an entire nation (Israel). Now, Jesus has invited
the entire world to become members. You shouldn't ignore the fact that He has never shrunk the number of people in Covenant with Him- rather, He has only
increased it in size. He has never changed His mind on
any group of people-
single or married, old or young- being numbered among His faithful, and you need to realize this.
During His time with Israel, the Nation-Covenant, God called for parents to have their sons circumcised so that they could enter into this Covenant. He was willing
then to admit the youngest among us into His Covenant- what exactly do you think has happened that made God turn back on this? And why did it take over a millenium for people to "get it"? Though the circumcision made by hands is no longer to be observed, the circumcision made
without hands, which is baptism-
is. Baptism is how
all of us can enter into that Covenant, and it is modeled after its Old Covenant precursor, circumcision.
There is a reason for the New Testament, but it's
not to contain every single teaching we must adhere to. Were that the case, perhaps Jesus would have taken your example.
You believe that Jesus would have had to be baptised as an infant in order for it to be a valid teaching. According to your logic, if Jesus wanted the elderly to be baptized, He would have been baptised as an old man. He wasn't baptized as an elder, therefore (according to your own logic), one cannot be baptized if they're older than Jesus was at the time of His baptism.
If Jesus wanted the New Testament to contain
all of His teachings, He would have told His disciples to write as much in even
one of their letters. He would have told John, "Don't conclude your gospel the way you did- it suggests that a book alone cannot contain all that I have taught". If He desired for Scripture Alone to be a satisfactory rule of faith, He wouldn't have allowed Paul to refer to the Church (which He founded on a visible leadership, as He had done for Israel) as the pillar of truth rather than Scripture. And lastly (for now), He wouldn't have allowed Christians to hold to a false belief of Scripture, Tradition and Magisterium for 1500+ years until the Baptists arrived on the scene.