Of course there might be legal reasons for excluding those below a certain age (though 18 seems high for this boundary) as they don't want their offer to be seen as employing minors.
For any company that offers bounties, my points are still relevant: not handing the out to a subset could encourage that subset to look for reward elsewhere, and the perception of labour use could be an important consideration.
Public relations has nothing to do with true facts, it has everything to do with how people interpret the details they see. If someone does mis-interpret and get on their high horse, others will join in without checking the facts themselves and it can be a nightmare trying to point out the truth of the matter over all that noise.
I think we need to start introducing limits on the height of horses :)