I've some experiences in freelancing. I guess this answer is correct because I've seen numerous job postings like this to give positive reviews. But one point to be noted is that, to stop this sort of rating manipulation, IMDB only counts votes from regular voters. That means if you just register and give couple of movies very high ratings, you votes are not going to be included in the calculation.
Anyway, I've another explanation, but I don't have any proof to back it up. Old movies are seen mostly by people who really love movies and passionate about them. They are more like movie critic type. On the other hand, new movies are seen mostly by general audiences. And I believe ratings from critics are most of the time less than the ratings from general audiences. So for this reason, being equal in quality, newer movies enjoy better ratings than old movies.