whats the cuttoff point?
8-bit?
the 1990's?
If I'm going to get all interlekshul about it, I'd say that it isn't actually so much a year or tech as it is a style.
Certainly pre-16 bit, games almost all came in a few distinct styles that you would recognise as old school. They tended to involve lives, brutal difficulty, score attacks and that kind of thing.
With 16 bit tech came the ability to massive increase the complexity of the interactive experience. They still made some old school games alright -- particularly in the early years -- but increasingly there was more to them than that.
Consequently, even the games that were old school in appearance (lives, platforms and so on) changed their nature. They became more forgiving of mistakes, started including save points, stopped having unforeseen deaths. It changed.
If I were to pick a particular year though, I'd probably go with something around 1988. It was shortly after that the likes of Zarch/Virus, Populous, Sim City and so on really revolutionised the whole concept of computer gaming.
There were definitely some modern games in amongst the earlier years though. Elite, for example, is totally a modern game.