This problem was offered by Prof. Jan Vanthienen who is one of the best known experts in decision tables and a member of the DMN Task Force. You can see other vendor's solutions to this challenge here: https://dmcommunity.wordpress.com/challenge/challenge-jan-2016/
Next month Prof. Vanthienen will provide his comparative analysis of different submitted representations along with his explanations of what constitutes a good decision table.
Solution #1 shows how the problem can be solved using a single decision table to represent all the combinations of age and years of service that are referred to in (or implied by) the rules.
See also solution #2 which splits the decision into two parts: (a) determining eligibility for vacation and (b) calculating the actual number of days. This is a pattern that is often seen in welfare eligibility systems.
Make these changes to your rules