The standard answer is that things gradually wear out. Also a lot of diseases are more common as you get older, especially cancer but also cardiovascular disease (problems with the heart and blood vessels). Some people believe that damage to the body from free radicals kind of adds up over time, causing ageing. There is a lot of research on ageing, and there are some people who believe people could live quite a lot longer then we do now – one guy I’ve heard giving lectures about this is Aubrey de Grey.
Of course, current AVERAGE ‘life expectancy’ is already a lot longer than it used to be. Although a few people used to live to be very old say, 200 yrs ago, most people in those days didn’t make it to the current retirement age of 65. Even when my dad was born in 1931, average life expectancy for a man was about 60 (my dad ‘beat’ this average, as he is 81 and still going). That same average number for a male born in 1998 was about 77, and for a boy born today it would be about 80. So you’ve got a good chance at making 100.
Apart from ‘wearing out’, there is evidence some of the progressive damage involved in ageing comes from processes that are made worse by things like unhealthy eating and being overweight. So if you want to live as long as possible, the message is stay fit, eat healthily and try and keep the weight off. I have to admit I’m not doing a great job with any of those myself.