Well, in "real life" it would be size. But that just means the party will go after fat orcs instead of dangerous ones.
What you could do is rule that the critter in question has some special property related to age. For example the tusks always get larger or whatever. Or you could save the special bounty for "named" boss monsters. Or you could just throw fate to the wind and have a special orc tooth table and the party would just hope you rolled "00" for a big one.