Best way to tell how old a Macaw is is to cut it in half and count the rings ... hehehehe ... (I guess I shouldn't joke like that someone might take me seriously) ...
Though I am unsure how to tell how old a macaw is I believe there may be a way to tell by their beaks ... but again, I'm no medical expert, I just play one on forums. But what I can tell you about age is this ... saying that a macaw is 4 years old is like saying a 4 year-old human ... these birds are long lived ... and if you want to give a your new macaw a forever home ... you better be prepared.
I am reminded of a Bolivian Blue and Gold macaw that I had the pleasure of meeting, his name was Sam. Sam had been with his original owner since she was 16 (he was her sweet 16 birthday gift) ... I met Sam last year when his original owner had to give him up because she had developed Rheumatoid Arthritis and couldn't really handle him any more ... SHE WAS 80! That made Sam at least 64 years old ... and oh my goodness, he was a love.
So, these birds will out live you if you give them the care, routine and non-routine, that they need, and that these awesome, awesome creatures deserve.