Rest Less, which offers advice to over-50s on getting the most from life, said the number of cases reached 3,668 in 2020.
It said its analysis of official statistics suggested that age discrimination saw the biggest annual increase of all complaints.
Rest Less predicted that the number of age discrimination complaints is set to increase further in the coming months because of large numbers of redundancies among the over-50s last year.
Founder Stuart Lewis said workers in their 50s and 60s have had a challenging time in the labour market over the last 12 months.
He added: “Unemployment levels soared by 48 per cent year on year and redundancies amongst the over-50s hit an all-time high in 2020.
“Additionally, with more than one million workers over the age of 50 still on furlough, and business concerns around the potential for new virus variants to delay reopening, we fear a new wave of redundancies may be on the horizon.
“We know that the pandemic has exacerbated age discrimination in both the workplace and the recruitment process.
“Age discrimination is unfair, unacceptable and has long-term damaging consequences on both the individuals involved and wider society.
“It needs to stop.”
Rest Less, which describes it self as the UK’s fastest-growing digital community for the over 50s, provides advice on issues ranging from health, lifestyle, learning and finance to volunteering.
Patrick Thomson, of the Centre for Ageing Better, said: “As the labour market adapts to the unwinding of furlough, reopening of some businesses and closing of others, many older workers are being caught in the middle.
“Employment tribunals are often the last course of action for people facing discrimination or unfair treatment in the workplace, and it is worrying to see so many older workers needing to pursue them.”
The Equality Act of 2010 includes provisions that ban age discrimination against adults in the provision of services and public functions