I'm not sure I personally believe in hell at all to begin with. That being the case, age is irrelevant.

But if we stick to a more fundamentalist understanding of scriptures then there is one easy and clear answer. Infants are incapable of choosing Christ, therefore they go to the same place all others who fail to choose Christ and die under the yoke of original sin go, hell.

If you aren't a Calvinist this sounds harsh and unfair. But if you come from the reformed tradition, then it's really no different from any other person who ends up in hell, they are there because they simply weren't among "the elect"; God did not sovereignly pre-ordain them to be among those whose names would be written in the book of life, it is not more unfair for a child than it would be for an adult.

However, there is a glimmer of hope, even for a fundamentalist. In speaking of John the baptist, the bible tells us:

Luke 1:15 for he will be great in the sight of the Lord. He is never to take wine or other fermented drink, and he will be filled with the Holy Spirit even before he is born.

So, it is at least possible for someone to be filled with the holy spirit even before being born. Is this an exceptional case or even one of a kind? Maybe...but it at least means it is possible.