New Products - what is the age cutoff?

The setting Admin > Configuration > Maximum Values > New Product Listing - Limited to … determines what “New Product” means:

Limit the New Product Listing to
0= All Products
1= Current Month
7= 7 Days
14= 14 Days
30= 30 Days
60= 60 Days
90= 90 Days
120= 120 Days

Reducing this value means a more efficient query for generating a list of new products.

ALTERNATIVE: If, however, you wish to hand-pick which items show up, then turn OFF the New Products box, and use Featured Products instead. See Admin > Catalog > Featured Products to pick the products.