respond 300 words:

There would be a unidirectional hypothesis with
Ho: Older same number of accidents or more than younger.
Ha: Older fewer accidents than younger.
The rejection region at the 5% level is all on the left side of the curve (fewer accidents than younger.) There is a larger area for rejection, so the absolute value of the test statistic can be smaller to reject the null hypothesis than the non-directional.
Non-directional:
Ho: older accidents=younger accidents
Ha:; the two are different or not equal
This one has a rejection region on both sides, so the absolute value of the test statistic is greater than in the first one to be in the rejection region
Given that older people and younger people both have more accidents, I would favor the nondirectional hypothesis which will look for a difference between the two. That is generally the preferred way to go, unless there is a strong suggestion one is better than the other and the study was set up that way.