Is it possible for a non-Muslim (one who believes in multiple gods) to enter Paradise if he died before reaching the age of forty, was upright and of good character, especially since it is said that whoever dies before the age of accountability (age of forty) is forgiven?

This statement is incorrect, for Islam has closed all paths to Paradise except through the Prophet Muhammad, peace and blessings be upon him. Whoever dies as a polytheist after the proof has been established against him will be an inhabitant of Hell, abiding therein eternally. Allah the Almighty says: "And whoever desires a religion other than Islam, it will never be accepted from him, and in the Hereafter, he will be among the losers." (Quran 3:85). And Allah the Almighty does not forgive that partners be associated with Him. He says: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly gone astray in far error." (Quran 4:116). The Prophet, peace and blessings be upon him, informed us that whoever, whether a Jew or a Christian, hears of him and does not believe in him, will be an inhabitant of Hell. Allah has warned disbelievers of eternal abode in Hell, saying: "Indeed, those who disbelieved and committed wrongdoing - never will Allah forgive them, nor will He guide them to a path. Except the path of Hell, wherein they will abide forever. And that is ever easy for Allah." (Quran 4:168-169). The messengers were sent to affirm the Oneness of Allah and to dedicate worship solely to Him. Allah the Almighty says: "And We did not send before you any messenger except that We revealed to him that there is no deity except Me, so worship Me." (Quran 21:25). Whoever dies as a polytheist after reaching puberty is a disbeliever and an inhabitant of Hell, abiding therein eternally, and his good qualities in this world will not benefit him. Puberty is reached upon completing fifteen years of age, or the growth of pubic hair, or the emission of semen; for a female, it also includes the onset of menstruation. Whoever dies upon disbelief before reaching puberty, his matter is with Allah the Almighty.