Will someone who was born a Muslim, then grew up searching for the true religion, and died before discovering that Islam is the true religion, go to Paradise or Hell?

First: Whoever dies a monotheist Muslim is among the people of Paradise, even if they are punished for their sins before entering it. And whoever dies a polytheist, disbelieving in Allah, will be in the Fire, abiding therein forever and ever, in accordance with His (Allah's) saying: (Indeed, whoever associates partners with Allah, Allah has forbidden him Paradise, and his abode will be the Fire) and His saying: (If you associate partners (with Allah), your work will surely be annulled, and you will certainly be among the losers), and the saying of the Prophet (peace be upon him): (Whoever dies associating partners with Allah in anything will enter the Fire). And whoever does not declare the Jews and Christians disbelievers, or doubts their disbelief, is a disbeliever by the consensus of scholars.

Second: Allah does not punish anyone until the message of their Lord reaches them, in accordance with His (Allah's) saying: (And We would not punish until We send a messenger), and the saying of the Prophet (peace be upon him): (By Him in Whose Hand is the soul of Muhammad, no one from this nation, neither a Jew nor a Christian, hears of me, and then dies without believing in that with which I was sent, except that he will be among the people of the Fire).

Third: If the Sharia has reached someone and the evidence has been established, then the one who disobeys has no excuse. The Imams of Fiqh (jurisprudence) have held that no respite should be given to a disbeliever against whom the evidence has been established. Whoever has received the Qur’an or an explanation of its meaning from the callers to Islam and persists in their disbelief is among the people of the Fire. And whoever lived in lands where the call to Islam is heard and did not believe, or Islam reached him and he understood what reached him and persisted in disbelief, or believed in a religion other than Islam after the Qur’an and Sunnah reached him, is a disbeliever.

Therefore, the person mentioned in the question is an apostate. An apostate is not given more than three days' respite. If he returns to Islam, otherwise he is to be killed. If he dies as a disbeliever before that, he falls under the rulings of disbelievers: he is not to be washed, nor prayed over, nor buried in Muslim cemeteries, and his wealth is not inherited. However, if Islam did not reach him correctly, or he has an excuse with Allah, then his matter is with Allah.