Yeah but that doesn't fit with the idea that the gospel is an instrument essential for salvation,if it can be excluded from whole age groups of people. Mercy and justice cannot be administered at the same time toward sinful creatures. If you opt for mercy then you concede that God is judge over salvation,if you opt for justice than these people,whoever they are must be punished for not accepting Christ and we know they wouldn't because they didn't hear the gospel,if you want to be consistent.
My belief system is that God saves who he chooses without conditions or partnership from the inferior creation,so I do not have this dilemma of ''need to hear the Gospel'' but then back down from this position in tricky situations like infant fatalities.
The Bible says that anyone who believes will be saved.... so no groups are excluded from salvation they just need to believe in Jesus Christ.
that gives us the issue that you seem to perhaps be asking and why I think there is an age of accountability (which is different for each person).
one needs to be able to comprehend enough to be able to believe.
can babies believe if all they know are the need to be fed and changed.
can unborn babies believe if their brain is nor even fully formed yet?
can the intellectually limited fully comprehend more than God is love.
just as a Father does not expect a 2 year old to know how to drive or to have the capability to learn how yet, so I believe God know those He has created in detail and knows when they can believe. God is just in that His judgments are righteous. He treats each person as they are only He knows the point at which a person is capable of belief.
So I do not believe babies are yet capable of belief.
I do believe the age of accountability is quite low though. some very young children have an understanding of God that few adults have. God talks about having the faith of a child, we are instructed to have this type of faith. and it is amazing how a child will simply trust you as an adult isnt it really. their trust or faith is something very precious.
God wants us to have that kind of trust/faith in Him.
faith comes by hearing and hearing by the word of God. there is an instruction in the new testament to 'have faith in God' - it is our choice to do that or not, as well as God giving us faith. I think God gives us faith by such things as intervening in our lives at times showing us He is real, giving us creation that declares Him, perhaps something like providing a verse on a wall somewhere that may speak to us as a non believer and cause us to ponder on the existence of God... thus growing faith bit by bit.
And God does such things with all people. What we do with such things matters, do we dismiss it or not. Everyone is given opportunity to believe or not believe.
That is why with evangelism God knows where each person is at, we cant assume where a person is. They may be very near to believing in Christ for salvation taking that step. or they may need to know a bit more or ponder a bit more first. or they may be hearing for the first time that there is a God who loves them. every person is different.
God knows this and is Merciful to ignorance (thankfully!). And He is Just in that He judges each person according to that persons journey.