what does the item's 'age' stat mean?

[John . H]

when I send a cmd 'stats items'
I got

STAT items:1:number 12
STAT items:1:age 5904
STAT items:7:number 1110100
STAT items:7:age 11065
STAT items:33:number 100
STAT items:33:age 6150

And I want to know what does the age mean?
Thanks for all tips.