Question

When did Pokemon NPCs start giving items?

Comments

They've been giving out items since the first Pokemon games.

Answer 1

Since Pokemon RBG, when Prof Oak gave one of the three starters, if I had to guess :P

That said, I'm pretty sure that was a feature since the beginning (don't quote me on that), and it was to promote game players to interact with the overworld for small rewards.
If the gym leaders gave TMs/HMs or usable items, it makes sense that the other NPCs do too, for example the nuggets obtained by defeating Team Rocket grunts on Nugget Bridge.

Answer 2

NPCs started giving you items In Pokemon Red, Blue, and Green.

In Pokemon Red, Blue, and Green, the first NPC to give you an item, would be the guy on route 1 that gives you a potion. He is the one that talks about how you should go to the Poke-Mart.