Using SOQL apex bind variables for date literals (e.g. LAST_N_DAYS, TODAY, THIS_QUARTER)

Wow, that's an interesting one! I doubt it's allowed though (at least without having to use Dynamic SOQL).

You can always do this instead:

Date d = System.today() - 7;
List<Opportunity> opportunities = [SELECT OwnerId, Amount, Probability FROM OPPORTUNITY 
where Amount > 0 and LastModifiedDate < :d];

You can use Database.query() with a string like:

Integer lastDays = 7;

List<OPPORTUNITY> opplist = Database.query('SELECT OwnerId, Amount, Probability FROM OPPORTUNITY where Amount > 0 and LastModifiedDate < LAST_N_DAYS:' + lastDays);

System.debug(opplist);

This is referred to as Dynamic SOQL.