Set x-axis to be at y=0 ggplot2

I am trying to get my plot's x-axis to stay at y=0 despite the y-axis limits being set to -0.50, 0.50. I can't seem to figure out a way to draw the axis myself while still using ggplot2...

See sample code here:

c_sdata <- summarySE(fa_data, measurevar = "cbias", groupvars = c("AGE", "Image_Type"))

# Use Image_Type as a factor as well as AGE
c_sdata$AGE <- factor(c_sdata$AGE, levels = c("2","1"))
c_sdata$Image_Type <- factor(c_sdata$Image_Type)

# Error bars SEM
ggplot(c_sdata, aes(x=Image_Type, y=cbias, fill=AGE)) +
  geom_bar(width=0.5, position=position_dodge(.6), stat="identity") +
  geom_errorbar(aes(ymin=cbias-se, ymax=cbias+se),
                position=position_dodge(.6)) +
  xlab("Stimulus Type") +
  ylab("Response Bias") +
  scale_fill_manual(breaks = c("2","1"),
                    name = "",
                    labels=c("Older Adults", "Younger Adults"),
                    values = c("grey57", "grey75")) +
  scale_x_discrete(limits = c("Upright_Faces", "Inverted_Faces", "Chairs", "Houses"),
                   labels = c("Upright Faces", "Inverted Faces", "Chairs", "Houses"),
                   position = "top") +
  theme_classic() +
  theme(legend.position = c(.25,.1),
        legend.direction = "horizontal",
        legend.key.size = unit(12, "points"))

Here is the output graph:

Current graph output

What I want is the x-axis to be at y = 0.