The claim boils down to
...every boy 15 and over killed in drone strikes now is automatically listed as an enemy combatant...
Most drone strikes have been performed so far the US or UK military. So I guess the claim might be about some of these.
Also it is unclear (even from context) which list is meant specifically. There is no complete official list of drone strikes from the US or UK military. Most information comes from NGO organizations like dronewars or reprieve which in turn rely partly on non-exhaustive, summarizing official reports of government agencies, partly on local media reports after drone strikes took place.
For example, the UK ministry of defense seems to use the terms insurgent, civilian, child in their reports of drone strikes. They do not specify the age or role of the killed people, instead just count them as fatalities (civilian or combatant).
According to the Bureau of Investigative Journalism US officials or spokespersons are quoted speaking about air strikes against "individuals threatening the force" which might be seen as a synonym for combatant. The possibility of hitting individuals not threatening the force seems to be not discussed very often from an official side.
A report in the NY Times discussed a disparity of the number of civilian casualties from drone strikes and another article there mentioned that "all military-age males in a strike zone [are counted] as combatants, according to several administration officials". However, the article fails to name the administration officials although it could explain the usual low number of reported civilian casualties. However, this could also be explained by really precise drone weapons, difficulties in determining the status of drone strike victims or deliberate misinformation.
The report on Counting Deaths from Drone Strikes created by the Columbia Law School concludes that "the uncertainty about civilian deaths is largely due to the U.S. government’s resistance to openly
providing information about strikes.".
All in all, I conclude that most official reports available do not specifically mention the status of a drone strike victim and while there is an indication that the US government might count all "military-aged" male victims as combatants (which would make the claim true) it is not sure because there is not enough available information about the details of the process of officially determining civilian and military casualties as well as the total count. It may be plausible, but we don't know for sure.