A report issued by the Women and Equality Committee last year found that the UK faces acute challenges recruiting and retaining an experienced, skilled workforce and is wasting the talents of more than one million people aged over 50 who would be willing to work if the right opportunity arose.
It concludes that age discrimination is the "root cause" of this problem.
Acas has published new guidance to help employers avoid discriminating against older workers and job applicants.
The 27 page document explains the law and sets out the most common situations where age discrimination might occur, including recruitment, promotion and performance management.
In common with other Acas guides, it differentiates between what employers have to do to comply with the law and what is good practice.
Age discrimination, also commonly called ageism, is one of the most common forms of unfair treatment at work. The age gap between staff can now be 50 years or more.