Default Useful life value for Asset Class

Hi,

I have created a new set of Asset Classes.

Now that i want to enter default values for these Asset Classes.

The default value which i want to assign is the (Planned useful life in years) , which i could make out that, it can be done in Tcode OAYZ. But the field is disabled in the tcode OAYZ to enter some default value. What could be the reason.

Is there anything else to be done for this field to be enabled in tcode OAYZ.

Regards

Gopi