Matt Damon finds it a lot harder to stay in shape as he gets older.

The 44-year-old actor, who is currently shooting the upcoming fifth installment of the 'Bourne' films, was forced to give up some of his favourite indulgences, including ''burgers and beer,'' to get ripped for his return to the franchise and admits he wouldn't do it if he wasn't being paid.

He said: ''It's a lot of work. You know what somebody said to me? They said, 'It's a lot of no's and not one yes.'

''And that's what it is. It's for work. It's the only time I would ever do it is for work. I like to say yes too much. I like burgers and beer. It's been no burgers and beer for a while.''

The movie's producer Frank Mashall recently shared a shirtless picture of the actor's chiselled physique from the set, but Matt claims he had to spend a lot longer in the gym to achieve the results compared to when he starred in 'The Bourne Ultimatum' in 2007.

He told E! News: ''It's painful. It's a different thing. I'm 44 now and I feel it, definitely.''

But the 'Martian' star, who raises daughters Alexia, 16, Isabella, nine, Gia, seven, and Stella, four, with wife Luciana, insists he is looking forward to celebrating his 45th birthday with his family next month.

He said: ''We'll probably go have dinner. [I just want] everyone to be happy.''