Why would kids be able to talk to spirits more?
They're less resistant to magical influence than adults. So when a spirit is talking to them they're more likely to hear it.
It doesn't matter if there's a reason why kids can talk to spirits, we can't have it in. If we did, then we'd either have to make our protagonist a child or come up with some complex and totally unnecessary reason why our protagonist can talk to spirits even though she's not a child.
Not true, the player characters simply found a way to make themselves more in tune with the spirit world and thus they're able to interact with spirits past childhood.