My wife and I have read the whole Left Behind Series and in many parts, they have taken a great liberty in their attempt to interpret “what may happen” during the Great Tribulation when the Antichrist is forcing people to receive the mark of the beast. I think your question is an example of “going beyond Scripture” and by doing so, they are actually “contradicting Scripture.” Let’s remember, even though the authors of the Left Behind Series were earnest believers, their books are still considered FICTION, even though they seek to base their novels on Scripture. What do I think? Again, I believe they have contradicted Scripture, so I believe this example (of a 17-year-old believer having the “mark of God on his forehead” and the “mark of the beast on top of that”) is an impossibility. Scripture alone is our AUTHORITY, and we must not “add or subtract to the Word of God.” If Scripture is clear on any given subject, we can be dogmatic in our interpretation, and we must reject any teaching that contradicts it.

In Revelation 13:11-18 we read, “Then I saw another beast coming up out of the earth, and he had two horns like a lamb and spoke like a dragon. And he exercises all the authority of the first beast in his presence and CAUSES THE EARTH AND THOSE WHO DWELL IN IT TO WORSHIP THE FIRST BEAST, whose deadly wound was healed. He performs great signs, so that he even makes fire come down from heaven on the earth in the sight of men. And he DECEIVES THOSE WHO DWELL ON THE EARTH BY THOSE SIGNS which he was granted to do in the sight of the beast who was wounded by the sword and lived. He was granted power to give breath to the image of the beast, that the image of the beast should both speak and CAUSE ALL, both small and great, rich and poor, free and slave, TO RECEIVE A MARK ON THEIR RIGHT HAND OR ON THEIR FOREHEADS, and that no one may buy or sell except one who has the mark or the name of the beast, or the number of his name. Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666.” 

I have capitalized the words that teach us that “ALL who receive the ‘mark of the beast’ are DECEIVED and FORCED into doing so.” The Antichrist, through “signs and lying wonders” (see 2 Thessalonians 2:8-9), will be so persuasive that people will take the mark of the beast “of their own free will.” In the Left Behind Series, they would make you believe that “the FATHER MADE THE SON to take the mark of the beast.” This is a clear contradiction of what we have just read. The truth is every believer will REFUSE to take the mark of the beast, no matter who may be trying to influence them to take it.

Now let’s read Revelation 14:9-11, “If ANYONE worships the beast and his image, and RECEIVES HIS MARK ON HIS FOREHEAD OR ON HIS HAND, he himself shall also drink of the wine of the wrath of God, which is poured out full strength into the cup of His indignation. He shall be tormented with fire and brimstone in the presence of the holy angels and in the presence of the Lamb. And the smoke of their torment ascends forever and ever; and they have no rest day or night, who worship the beast and his image, and WHOEVER RECEIVES THE MARK OF HIS NAME.” Again, God’s Word is clear; ANYONE who receives the mark of the beast will experience God’s wrath and be tormented forever and ever. We do not see any exceptions in this passage. We do not read of the possibility of a true believer in Christ receiving the mark “because his father made him do it” and then escaping God’s wrath. There may be people who “profess to be saved” before the mark is implemented and in time, they give up their profession of faith in Christ and take the mark of the beast. But a true believer will NOT be deceived by the Antichrist, and he/she will NOT take the mark of the beast.  (DO)  (735.1)