I think 'old skool' is subjective and different people will have different views on what it is and isn't...
For me, 'old skool' isn't about age / time - it's about style, and is the same for most genres, not just hip hop.
I think it's a way of referring to a style of music that is representative of what that genre was like when it first emerged / became popular. Alot of genres change over time, look how different hip hop is today compared to the late 80's / early 90's - same can be applied to some of the dance genres i.e. trance, drum and bass.
These might not be the best examples/explanation, but to me, old skool hip hop would be something whereby the lyrics were meaningful, not just bragging or abusing and the sound had some kind of melody / rhythm.
Meaningful lyrics were quite different, NWA and Public Enemy had attitude, but the lyrics also had meaning, other artists like Tribe Called Quest, De La Soul, Gangstarr, Young MC etc were less aggressive, but still had lyrical meaning.
Others will probably have different opinions, this is just my take on 'old skool' and how I would apply it to hip hop..