Starter Tools

    public static void PlayerJoinedWorld(EntityJoinWorldEvent event)
    {
        PlayerEntity player = (PlayerEntity) event.getEntity();
        LazyOptional<IPlayerHandler> playerc = player.getCapability(PlayerProvider.CAPABILITY_PLAYER, null);
        IPlayerHandler player_cap = playerc.orElse(new PlayerCapability());

        if (!player_cap.joinWorld())	{

            player_cap.setjoinWorld(true);
            player.addItemStackToInventory(new ItemStack(ItemInit.KUNAI.get()));

            Biome biome = player.getEntityWorld().getBiome(player.getPosition());
            String biomename = biome.getDisplayName().getString();
            player.sendMessage(new StringTextComponent("You were left at this " + biomename + " as a baby."));
        }
    }

The above is a snippet from my own mod, that being said I was using capabilities. 

I simplified the above to below;

    public static void PlayerJoinedWorld(EntityJoinWorldEvent event)
    {

        if (!player.joinWorld())	{

            player.setjoinWorld(true);
            player.addItemStackToInventory(new ItemStack(ItemInit.KUNAI.get()));
        }
    }

The variable "player" must retain over game sessions, and preferably worlds (capabilities does both). With that being said, I think @ShockingArtist has great procedures for you to have a look at.

you could just use procedures and nbt tags...