There is no year in which pollution started.
If you call something “pollution” depends on the type of substance (which molecules), concentration and the resecptors that have to deal with these substances (animals, humans, plants, etc.). So I would say the way how pollution appears nowadays is the result of the development of humanity of the remaining 100s and 1000s of years.
Humans have always polluted the places they have lived. So pollution is the’ presence in or introduction into the environment of a substance which has harmful or poisonous effects’. To break that down if you add dead animals or leftover food to the water, you can pollute the water source. If you burn or weld iron or copper like the celts or Vikings, the compounds produced could leach into the soil and kill off plants. The issue today with pollution is that there are a) more people so more pollution and b) we use a lot more disposable/once off things, such as paper plates or plastic packaging on food.