Minimum level to get decent exp at cows hell?

I believe it's level 60

Level: 81

http://classic.battle.net/diablo2exp/monsters/act1-cow.shtml

https://diablo2.diablowiki.net/Experience#Characters_Above_Level_70

Characters Level 25-69

From level 25 to 69, 100% of experience is awarded for monsters within 5 levels of a character. Good experience can also be obtained from monsters higher than a character's level:

EXP*(Player Level / Monster Level).

Experience gains are penalized when the monsters are more than 5 levels below the character.
lvl diff exp%
6
7
8
9
10
>10 81%
62%
43%
24%
5 %
5 %

As with characters below level 25, monsters more than 10 levels below a character's level are worth just 5% of their potential experience.

The same ratio of experience rewards applies to characters over level 70.