Old units not earning XP from battles any more

I have a few crossbows that could be upgraded to Gatlings, but I wanted to hold off until I got them range promotion (next promotion). The crossbows are very close to the promotion – maybe 15XP left to get – but they don’t seem to earn XP any more.

Is there something that will stop them earning XP, like the era I’m in, or the fact that these units are now out-dated?

This may be a stupid question, but are you killing barbarian units? Those stop granting XP to units Level 3 and up.

Doh!
Not a stupid question (or maybe it is and I'm the stupid one). I was barb-hunting, and they are at level 3.
Man - just wasted about 20 turns embarking these xbows to some poxy island to kill a nest of barbs, and all for nought!

Thanks for the lesson!

Well, if you like, you can always find some mods to raise or remove the limit on XP gained from barbs.