Possibly. Although the legislation bans different treatment on grounds of age, there is a general exemption where:
- the award or increase in a benefit is meant to reflect a higher level of experience, reward loyalty, or increase motivation; AND
- the employer expects business benefits from the above; AND
- the criterion (whatever it is) is applied similarly to all staff in a similar situation.
If you believe that the business will in future benefit from the experience and loyalty of your longer-serving employees, it may therefore be both reasonable and lawful to allow them the pick of the holiday dates. If, however, you allow Robert and Claudine the pick of the holiday dates, but insist that Jack and Mary, though equally long-serving, must vie with everyone else on a first-come, first-served basis, you might well fall foul of the legislation.
In general terms, it would be advisable to review all your pay and benefits, if you haven't already done so, to ensure they comply with the requirements of the age discrimination legislation.