The passage in Ephesians 1:4 addresses a personal pronoun, not a plan: US.If God has already planned who would be His own then what was the whole purpose of Christ's life, death and resurrection.
Even though God created man, yet Jesus being before the foundation of the world had His purpose in the world as God had all things planned out before hand. Do you think God would have known man would sin against Him? I would think so because of the fall of Satan before man was ever created. Like Judas, Jesus told him to go do what he needed to do as Jesus already knew Judas would sell Him out. God already knew ahead of time as Jesus is the arm of God even before the foundation of the world, Isaiah 53:1. This is why Mary, even though not born yet, was already predestined before the foundation of the world for God's purpose here on earth, Jeremiah 1:5.
God does not select before the womb those who will spend eternity with Him or those who will be cast into the lake of fire. God is a sovereign God that in his love and patience for us would not want anyone to perish and only God knows the intents of the heart as he judges us, 2 Peter 3:9. God wants all of us to come to repentance, but not everyone will for the way they are raised from birth. Many babies are born into families that have rejected God and will be raised to also come against Him as their hearts become hardened unto Him. Although, a hardened heart can be changed if given proper nourishment.
Age of accountability is nowhere found in the Bible and is a preconception that only leads to speculation. Infant baptism is also unscriptural as a baby can not know faith and regeneration as this takes place at the same point in time. A baby can not be covered by the blood of Jesus until they come into the knowledge of good and evil and accept Him as Lord and Savior. Many use Ephesians 1:4, 5 to support this, but that is not what these verses are saying. These two verses are very similar to that of 2 Peter 3:9 that through Gods love he breathed his breath (spirit) into us when we first came from the womb making us a living soul and that he would want us to be holy and without blame. Staying holy without blame is through repentance and acceptance that brings us to that adoption that is in Christ Jesus thus we were made for Gods good pleasure.
Eph 1:4 According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
Eph 1:5 Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will
God did not predestine the man, but predestined the plan of salvation through Christ Jesus before the foundation of the world. We were created to be holy and without blame before Him.
Adam was created perfect until he fell to temptation.
God chose US in Him BEFORE the foundation (creation) of the world, that WE should be holy and blameless before Him in love.
The whole purpose of Christ was to sacrifice a Holy Son in Eternity for an (created) unholy being in time: Man. This is how I receive US being chosen BEFORE (in front of) then foundation (creation) of the world. It meshes nicely with Revelation 13:8 -
And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world. Rev. 13:8.
This book of life with the names of those for whom Christ would die for is prepared also before (in front of) the foundation (creation) of the world. That is predestination.
But I will also agree with you on this if you take it (the Plan) a little bit further: It was the Father's Plan (in Himself in Eternity), the Son implemented the Plan, and the Holy Spirit allies the Plan to God's Elect past, present, and future. You mention 2 Peter 3:9. Let me post it here now:
The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to US-ward, not willing that any [of US-ward] should perish, but that all [of US-ward] should come to repentance. 2 Pet. 3:9.
Since Peter is writing to "those who have obtained like precious faith with us" he is addressing believers ONLY. This also speaks of predestination.
If Adam was perfect before he sinned the conclusion would be that God reduplicated Himself in dirt, and/or sin comes from perfect (whatever that is for one is either good or evil, sinful or sinless.
And he said unto him, Why callest thou me good? there is none good but one, that is, God: Mt. 19:17.
You can insert for 'good' any noun you want: holy, sinless, good, righteous, etc. Jesus is saying there is only ONE God who is Perfect, there is NONE like Him (this includes Adam at his creation), and God gives His glory (of which Holy, Sinless, Perfect, Good, Righteous, All-Wise, All-powerful, All-knowing, and the other Deific Attributes of God only God possesses, not man. Adam was not perfect. IF Adam was perfect he would have to possess ALL the deific Attributes of God and he didn't.