At what age can a person accept Christ? Is there an age that is truly too young to do so?
There is nothing in the Scriptures one way or the other about a specific age that is appropriate or required for receiving Christ as Lord. And since Jesus exhorted us to receive the Kingdom "like a little child"* I assume the decision of a child to embrace Christ is no less meaningful than for an adult.
Having said that, I should add that regardless of the time of life in which one makes the decision to accept Christ, there is always the need for discipleship and instruction in the Word. That means under no circumstances should we ever place our confidence in someone's past decision for Christ and go on as if nothing more is needed.
I've spoken to many adults over the years who have shared with me how as children they opened their hearts to the Gospel, praying the sinner's prayer, but who later in life managed to walk away from following Christ out of neglect or a fascination with the world. In such cases it's not uncommon for these individuals to relate how they had little or no understanding of spiritual truths because they were never properly discipled or trained in the Word of God. As much as it pains me to say it, as often as there is a child praying to receive Christ as Savior, there is also an adult in their life thinking their job is now completed since the child has made a decision for Christ.
But a child of 5 or 6 years cannot be expected to grow and mature properly without guidance and instruction. In many ways a parent's job is just beginning when their child prays to receive the Lord.