The guide of this website about EV's says that it's better to train low level pokemon (+- level 20). So if I had a 2 pokemons with the same IV, nature, ... and I would EV-train one of them starting at level 1 and another one starting from level 50 (just caught), would the first one have higher stats than the second one?
You wouldn't know. It all depends on what Pokemon you battle. If Pokemon #1 fought a Pidgey and Pokemon #2 fought a Forretress, they would get different EVs. Assuming you battle the same Pokemon, the stats will be EXACTLY the same.