What age is Runeterra in?

That technological stuff mainly comes from Piltover&Zaun who are cities that manage to remain independent from other nations thanks to their extreme technological advantage.(and even then that technology uses magical crystals as their power source).

Piltover does seem to mainly keep their technology to themselves,which is why it isn't spread all over the globe,though some nations have shown to slowly adapt to the technological arms race(Noxus did use zaunite war machines against ionia and ionia did create a sniper rifle)

They're in an industrial era exaggerated my magic and mysticism. Anywhere similar to late 1600s to early 1800s in our world. Remember great technological and cultural disparities existed irl too, it's just that magic exaggerates that more in Runeterra.