The flush toilet as we know today was actually invented as early as 1596.
But although the flush toilet was invented in 1596 the flush toilets didn't see widespread production and use until the year 1851.
Even by the year 1851 it was more of the richer or higher class folks that had flush toilets in their homes.
Most people had flush toilets inside their houses by 1901 to 1935 but some people still used outhouses even then.
The more money the person had the better chance they had at having a flush toilet because not everyone could afford to have a flush toilet or afford to have the plumbing installed for the flush toilet.
So some people and families continued to use outhouses well into the 1930's and 1950s as well.