Sarah was 90 years old when she conceived Isaac 
Genesis 17:17: “Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?”

Sarah was 127 years old when she died
Genesis 23:1: “And Sarah was an hundred and seven and twenty years old: these were the years of the life of Sarah.”

From this calculation we can say with certainty that Isaac was at not older than 37 years of age at the time he went with his father for the sacrifice. But how old was he exactly?  Let’s take a look at the deeper meaning behind the actual event. 

Abraham represented God, our heavenly Father.  Isaac represented Jesus, God’s precious only son.  We know this based on all the similarities between God and Abraham and Jesus and Isaac as outlined below.

Abraham was willing to sacrifice Isaac
Genesis 22:2: “And he (God) said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering.”

God’s plan from the beginning was to sacrifice His dear son Jesus to save the world, because God loves the world of mankind
John 3:16: “For God so loved the world, that he gave His only begotten son…”
1 John 4:10: “Herein is love, not that we loved God, but that He loved us and sent His son to be the propitiation (meaning satisfaction) for our sins.”

Isaac had to carry his own wood for the sacrifice
Genesis 22:6: “And Abraham took the wood of the burnt offering, and laid it upon Isaac his son.”

Jesus had to carry his own cross
John 19:16-17: “Then delivered he him therefore unto them to be crucified. And they took Jesus and led him away. And he bearing his cross went forth into a place called the place of a skull, which is called in the Hebrew Golgotha.”

Isaac was to be the sacrificial lamb
Genesis 21:7-9: “And Isaac spake unto Abraham his father, and said, My father: and he said, Here am I, my son. And he said, Behold the fire and the wood: but where is the lamb for a burnt offering?  And Abraham said, My son, God will provide Himself a lamb for a burnt offering: so they went both of them together. And they came to the place which God had told him of; and Abraham built an altar there, and laid the wood in order, and bound Isaac his son, and laid him on the altar upon the wood.”

Jesus is referred to as “the Lamb” or sacrificial lamb:
John 1:29: “The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.”
Revelation 13:8: “…the Lamb slain from the foundation of the world.”
Revelation 5:12: “Worthy is the Lamb that was slain to receive power, and riches, and wisdom, and strength, and honor, and glory and blessing.”

Isaac did not fight/resist Abraham but allowed his father to tie him down for the sacrifice
Genesis 21:9: “And they came to the place which God had told him of; and Abraham built an altar there, and laid the wood in order, and bound Isaac his son, and laid him on the altar upon the wood.”

We know Isaac was at least of the age when he could have easily overpowered his elderly father.

Jesus did not fight/resist God’s plans for him, and he gave himself a willing sacrifice
Isaiah 53:7: “He was oppressed, and he was afflicted, yet he opened not his mouth: he is brought as a lamb to the slaughter, and as a sheep before her shearers is dumb, so he openeth not his mouth.”
John 19:9-10: “and (Pilate) saith unto Jesus, Whence art thou? But Jesus gave him no answer. Then saith Pilate unto him, Speakest thou not unto me? knowest thou not that I have power to crucify thee, and have power to release thee?”

Through the Bible, God ties thousands of years of history back and forth between the Old and New Testament.  He provided several examples of His plans for mankind through the willing sacrifice of His precious son Jesus.  This portion of the life of Abraham and Isaac is a beautiful picture of this.

So, how old was Isaac when he was offered for sacrifice? 
Isaac had to be strong enough to carry the wood for the sacrifice.  Isaac also willingly allowed himself to be bound and laid on the alter.  This would take a mature nature that both trusted his Father, but also trusted the God of his Father, and faith to allow himself to be sacrificed.  Considering these points and based on the scriptures above that Isaac represented Jesus, Isaac was likely to have been 33 years of age, as Jesus was at the time of his sacrifice. 

Additional Resources: 
Christian Questions Podcast 
Episode #1246: Does God REALLY Require Human Sacrifices?
Clarifying Abraham sacrificing Isaac, and if we should make oaths or not  
Preview Video  
CQ Rewind Show Notes

For children, parents and Bible class teachers: 
Animated video “Why did God tell Abraham to sacrifice Isaac?”