Can an employer ever justify discriminating on grounds of age?

In some circumstances, yes. Employers will not be liable for age discrimination where it is found to be objectively justified. This is a strict test based on EU law and they will need to show that their actions amounted to a legitimate aim, and were an appropriate and necessary means of achieving that aim. For example:

• In a direct discrimination case, an employer may try to argue that it was necessary for workforce planning purposes to advertise for workers under 50 because the age profile of their workforce was very heavily concentrated at a range close to retirement. They would need to demonstrate that other less discriminatory measures were not appropriate or reasonable. For example, simply actively encouraging applications from a wide variety of sources, and enabling their older workers to stay on past retirement might be a more reasonable way of solving management’s concerns about workforce planning.
• An employer may try to justify an indirectly discriminatory promotion eligibility system based on acquiring at least fifteen years’ experience with their company or in a particular industry thereby disadvantaging younger workers who would not have had the chance to gain as much experience. Justifying such a policy would be likely involve showing that carrying out the job concerned at the higher level actually required such a long period of relevant experience with the company or in that industry. It might alternatively be manageable and possible for the company to assess potential capability to function at the higher level in other, less discriminatory, ways rather than apply an arbitrary filter.