why do today fighters have short careers

[Sam Donald]

when in the first 30 years or so on the 20th century, they fought for years and years

[Sugarj]

I doubt its much different really. A decent world class fighter can often drag out a career over the larger part of two decades or more. Some, Like Roberto Duran and George Foreman can manage rather longer!

The vast majority of the pound for pound superstars of the last 50 years have had long careers:

Ali: 21 years
Holmes: +30 years
Leonard: 21 years
Hearns: +20 years
Hopkins: +20 years
Mosley: Circa 20 years
De la Hoya: 17 years
Jones jnr: +20 years
Holyfield: +25 years
Tyson: 20 Years

Pac and Mayweather have been fighting professionally over 15 years each and are still at the top of many pound for pound lists.

[catalinul]

Thing is fighters always fought past they're prime.

Then and now.Very few can stop at the right moment(it's not a case of knowing when to do it,they know but usually need money).

[rightsideup]

benard hopkins and james toney have been around awhile

[GrandpaBernard]

when in the first 30 years or so on the 20th century, they fought for years and years

Less skills today meaning many fighters today rely heavily and depend on athleticism which fades with age. If you have skills to fall back on, you can still use them when you're older